Optimize some header and request pipeline handling

client/client-http/src/main/java/software/amazon/smithy/java/client/http/HttpClientProtocol.java

@@ -36,13 +36,14 @@ public MessageExchange<HttpRequest, HttpResponse> messageExchange() {
     @Override
     public HttpRequest setServiceEndpoint(HttpRequest request, Endpoint endpoint) {
         var merged = request.uri().withEndpoint(endpoint.uri());
-        var requestBuilder = request.toBuilder();
+        var modifiableRequest = request.toModifiable();
+        modifiableRequest.setUri(merged);
 
         // Merge in any HTTP headers found on the endpoint.
         if (endpoint.property(HttpContext.ENDPOINT_RESOLVER_HTTP_HEADERS) != null) {
-            requestBuilder.withAddedHeaders(endpoint.property(HttpContext.ENDPOINT_RESOLVER_HTTP_HEADERS));
+            modifiableRequest.headers().addHeaders(endpoint.property(HttpContext.ENDPOINT_RESOLVER_HTTP_HEADERS));
         }
 
-        return requestBuilder.uri(merged).build();
+        return modifiableRequest;
     }
 }

client/client-http/src/main/java/software/amazon/smithy/java/client/http/plugins/UserAgentPlugin.java

@@ -5,7 +5,6 @@
 
 package software.amazon.smithy.java.client.http.plugins;
 
-import java.util.List;
 import java.util.Locale;
 import software.amazon.smithy.java.client.core.AutoClientPlugin;
 import software.amazon.smithy.java.client.core.CallContext;
@@ -78,10 +77,9 @@ static final class UserAgentInterceptor implements ClientInterceptor {
         @Override
         public <RequestT> RequestT modifyBeforeSigning(RequestHook<?, ?, RequestT> hook) {
             if (hook.request() instanceof HttpRequest req && !req.headers().hasHeader("user-agent")) {
-                return hook.asRequestType(
-                        req.toBuilder()
-                                .withReplacedHeader("user-agent", List.of(createUa(hook.context())))
-                                .build());
+                var updated = req.toModifiable();
+                updated.headers().setHeader("user-agent", createUa(hook.context()));
+                return hook.asRequestType(updated);
             }
             return hook.request();
         }

client/client-mock-plugin/src/main/java/software/amazon/smithy/java/client/http/mock/MockPlugin.java

@@ -242,7 +242,7 @@ public HttpResponse send(Context context, HttpRequest request) {
             // Track the request here rather than when the request is created because this method is called
             // more often than the request creation method when retries happen.
             if (trackRequests) {
-                requests.add(currentRequest.request());
+                requests.add(currentRequest.request().withRequest(request.toUnmodifiable()));
             }
 
             MockedResult result = null;

http/http-api/src/main/java/software/amazon/smithy/java/http/api/HttpMessage.java

@@ -150,8 +150,10 @@ default B body(Flow.Publisher<ByteBuffer> publisher) {
          * @param headers Headers to add and merge in.
          * @return the builder.
          */
+        @SuppressWarnings("unchecked")
         default B withAddedHeaders(HttpHeaders headers) {
-            return withAddedHeaders(headers.map());
+            headers.forEachEntry(this::withAddedHeader);
+            return (B) this;
         }
 
         /**
@@ -168,15 +170,26 @@ default B withAddedHeaders(HttpHeaders headers) {
          * @param headers Headers to put.
          * @return the builder.
          */
+        @SuppressWarnings("unchecked")
         default B withReplacedHeaders(HttpHeaders headers) {
-            return withReplacedHeaders(headers.map());
+            headers.forEachEntry(this::withReplacedHeader);
+            return (B) this;
         }
 
         /**
          * Replaces a header.
          *
          * @param name  Header name to replace.
-         * @param values Header value to replace.
+         * @param value Header value to replace.
+         * @return the builder.
+         */
+        B withReplacedHeader(String name, String value);
+
+        /**
+         * Replaces a header.
+         *
+         * @param name  Header name to replace.
+         * @param values Header values to replace.
          * @return the builder.
          */
         default B withReplacedHeader(String name, List<String> values) {

http/http-api/src/main/java/software/amazon/smithy/java/http/api/HttpRequestImpl.java

@@ -93,6 +93,12 @@ public Builder withReplacedHeaders(Map<String, List<String>> headers) {
             return this;
         }
 
+        @Override
+        public HttpRequest.Builder withReplacedHeader(String name, String value) {
+            modifiableHttpRequest.headers().setHeader(name, value);
+            return this;
+        }
+
         private void beforeBuild() {
             Objects.requireNonNull(modifiableHttpRequest.method(), "method not set");
             Objects.requireNonNull(modifiableHttpRequest.uri(), "uri not set");

http/http-api/src/main/java/software/amazon/smithy/java/http/api/HttpResponseImpl.java

@@ -81,6 +81,12 @@ public Builder withReplacedHeaders(Map<String, List<String>> headers) {
             return this;
         }
 
+        @Override
+        public HttpResponse.Builder withReplacedHeader(String name, String value) {
+            modifiableResponse.headers().setHeader(name, value);
+            return this;
+        }
+
         @Override
         public HttpResponse build() {
             return modifiableResponse.toUnmodifiable();