[Go SDK] Container Worker pool functionality.

[lostluck]

• Allows the Go SDK Container to execute as a SideCar worker pool from the default Beam Go container, with the default port of 50000.
• Enables use as a containered local worker pool as described here for the External configuration: https://beam.apache.org/documentation/runtime/sdk-harness-config/
  • All execution is pooled locally as per normal.
• Code can be shared to other SDK container boot loaders, since it largely just re-executes the current bootloader process in the container.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[lostluck]

R: @damondouglas

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers