use WindmillChannelFactory to control what types of channels to generate

[m-trieu]

WindmillStubFactoryFactory is an abstraction that allows GrpcDispatcherClient to regenerate stubs based on updates to StreamingGlobalConfig. Currently only the isolation channel options is checked.

Remove WindmillStubFactoryFactory and push down the abstraction to where we create the channels by overriding WindmillChannelFactory and registering a callback to global config handle that will reload the GrpcDispatcherClient stubs based on whether the config is consumed by the channel factory.

---

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.

[m-trieu]

R: @scwhittle

[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