Add configuration option to allow customizing the internal execu…

[zoewangg]

Motivation and Context

Customers with high-throughput applications may experience dropped metrics because the internal executor queue fills up faster than metrics can be aggregated. The warning "Some AWS SDK client-side metrics have been dropped..." appears frequently

This change allows customers to configure the internal BlockingQueue used by the CloudWatchMetricPublisher, addressing the gap.

Modifications

• Added taskQueue(BlockingQueue<Runnable>) builder method to CloudWatchMetricPublisher.Builder

Testing

• Added taskQueueSettingIsHonored() test that verifies the custom queue is used by spying on the queue and verifying
  offer() is called
• Existing tests continue to pass

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](https://github.com/aws/aws-sdk-
  java-v2/blob/master/docs/LaunchChangelog.md)

License

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