Adds the user agent appId metadata tag

[cenedhryn]

Motivation and Context

AWS SDKs have added the appId metadata tag which allows users to set an identifier for their application that is added to the user agent string for every request. This PR adds the feature to the AWS SDK for Java v2.

Modifications

• Adds appId as a client param in SdkDefaultClientBuilder
• Adds options for configuration appId in environment variables, system settings and config files, and a resolver for determining if any such system value exists.

Testing

• Unit testing for the resolver
• e2e functional testing for the overall functionality.

Screenshots (if appropriate)

Types of changes

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

Checklist

• [x] I have read the CONTRIBUTING document
• [x] Local run of mvn install succeeds
• [x] My code follows the code style of this project
• [x] My change requires a change to the Javadoc documentation
• [x] I have updated the Javadoc documentation accordingly
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed
• [x] 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

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