Add ExponentialDelayWithHalfJitter backoff strategy

Motivation and Context

Introducing a new backoff strategy that is used in the LegacyRetryStrategy for backing off for throttling exceptions.

This change mimics the previous behavior in which the EqualJitterBackoffStrategy was used as the throttling backoff strategy for RetryMode.LEGACY. See here and here.

This backoff strategy differs from the full jitter exponential strategy on a cap used for the minimum delay time. The full jitter exponential backoff strategy waits in the range

[ 0, exponential-delay ]

whereas this one waits in the range

[ exponential-delay / 2, exponential-delay ]

The idea being since we are retrying after a throttling exception we want to wait some amount that is not or close to zero.

Modifications

Testing

Screenshots (if appropriate)

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

aws / aws-sdk-java-v2