Pub/Sub Schema Transform Read Provider

[damondouglas]

This PR address #21412 by implementing a PubSubTransformReadProvider.

Due to repeatedly failing beam_PreCommit_Java tests that have nothing to do with code in this PR (See example), to validate this PR, I ran the following gradle tasks prior to submission. All passed with the exception of ./gradlew sdks:java:io:google-cloud-platform:check that failed due to invalid Google Authentication credentials and only with tests that have nothing to do with code in the PR.

./gradlew rat
./gradlew spotlessCheck
./gradlew sdks:java:io:google-cloud-platform:check
./gradlew sdks:java:io:google-cloud-platform:checkStyleMain
./gradlew sdks:java:io:google-cloud-platform:checkStyleTest

I would like to request the following to review this PR:
R: @pabloem

---

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

• Choose reviewer(s) and mention them in a comment (R: @username).
• 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.

[pabloem]

Run Java PreCommit

[pabloem]

lgtm