SigV4: Add host header only when not already provided

[vsudilov]

Motivation and Context

Our use case is to send SigV4 requests to a transparent proxy to S3. We need to keep the Host header in-tact, since we use that for virtual bucket routing. Without these changes, the sdk will not both send a request to a custom endpoint and sign against a custom Host header.

No existing issue for sdk-v2, however this issue was reported and closed as a wontfix in sdk-v1: aws/aws-sdk-java#2974

Modifications

Exit early from SignerUtils#addHostHeader if a Host header is present in the request.

Testing

mvn install on localhost; all tests pass except:

Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:3.1.2:test (default-test) on project sqs

Example failure, seems completely unrelated to this PR:

-------------------------------------------------------------------------------                                                                   
Test set: software.amazon.awssdk.services.sqs.batchmanager.SqsAsyncBatchManagerTest                                                               
-------------------------------------------------------------------------------                                                                   
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 2.194 s <<< FAILURE! -- in software.amazon.awssdk.services.sqs.batchmanager.SqsAsy
ncBatchManagerTest                                                                                                                                
software.amazon.awssdk.services.sqs.batchmanager.SqsAsyncBatchManagerTest -- Time elapsed: 2.194 s <<< ERROR!                                     
software.amazon.awssdk.core.exception.SdkClientException: Unable to load region from any of the providers in the chain software.amazon.awssdk.regi
ons.providers.DefaultAwsRegionProviderChain@2c3bd568: [software.amazon.awssdk.regions.providers.SystemSettingsRegionProvider@6dad9ae6: Unable to l
oad region from system settings. Region must be specified either via environment variable (AWS_REGION) or  system property (aws.region)., software
.amazon.awssdk.regions.providers.AwsProfileRegionProvider@1f53e9b8: No region provided in profile: default, software.amazon.awssdk.regions.provide
rs.InstanceProfileRegionProvider@67874771: Unable to contact EC2 metadata service.] 

Screenshots (if appropriate)

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license