WIP: Expose idle timeout settings for HTTP connection

[vparfonov]

This PR adds a new HttpClient::new_with_connection_config constructor to expose pool_idle_timeout and pool_max_idle_per_host settings from the underlying Hyper client. This new constructor is now used by the Splunk and Elasticsearch sinks to pass their specific connection configurations.
By default Vector used:

pool_config: pool::Config {
                idle_timeout: Some(Duration::from_secs(90)),
                max_idle_per_host: std::usize::MAX,
            }, 

Sample of configuration:

[sinks.my_sink_id]
inputs = ["local_logs"]
type = "splunk_hec_logs"
endpoint = "http://splunk-hec-splunk"
default_token = "xxxx"
connection.idle_timeout_secs = 30
connection.pool_idle_per_host = 50 

[openshift-ci]

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[jcantrill]

you should additionally add this config to the other http transport we support:

• loki
• http
• aws cloudwatch
• aws s3
• opentelemetry
• azure?
• GCP