fix: defaultAwsSecurityCredentialSupplier fetches aws-credentials correctly from credential-url

[vverman]

In issue #898 a customer was facing issues when IMDSV1 was disabled and only IMDSV2 was enabled.

This is because currently the request to the AWS credentials endpoint returns a string as a response:

response = "{\n  \"Code\" : \"Success\",\n  \"LastUpdated\" : \"2026-02-18T00:10:44Z\",\n  \"Type\" : \"AWS-HMAC\", ,\n  \"AccessKeyId\" : \"ASxxx" ... }"

But we were accessing its properties as if it were a json response which was returning a null value.

response.AccessKeyId <- This would return null.

Now we make a call to get a json response, which can now be used to fetch the fields correctly:

response = "{"Code":"Success","LastUpdated":"2026-02-18T00:10:44Z","Type":"AWS-HMAC", "AccessKeyId":"ASxxx", ... "}

AWS credentials successfully fetched.
Successfully fetched bucket metadata:

P.S.: This was tested with the google-auth-lib 10.6.0 with the @google-cloud/storage.