[YAML] Add the ability to pre-process yaml files with jinja2.

[robertwb]

This will allow Beam YAML to be used for various template applications.

---

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.

[robertwb]

R: @Polber @damccorm

[github-actions[bot]]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[robertwb]

isort wants to put jinja2 after apache_beam, e.g.

 import apache_beam as beam
+import jinja2

Not sure how to resolve that.

[robertwb]

RowCoderTest, YamlIntegration, and Docs issues seem pre-existing.

[damccorm]

Precommit failures look real? Its not failing on master, but I also don't see any relevant changes here - https://github.com/apache/beam/actions/workflows/beam_PreCommit_Python.yml?query=event%3Aschedule

[robertwb]

A merge to master seems to have resolved the row coder issues. Running a full suite of tests again.

[damccorm]

(resolved some conflicts and cleaned up CHANGES.md while at it)

[damccorm]

Oh whoops, missed https://github.com/apache/beam/pull/30976/commits/91d4dc1af683c87808048bc03ed180522b12c13d - thanks for catching

[robertwb]

So I'm still trying to figure out why these same tests pass in isolation ( https://github.com/apache/beam/actions/runs/8722508185/job/23928542728?pr=30976 ) but fail when all tests are enabled ( https://github.com/apache/beam/actions/runs/8724953372/job/23936757826?pr=30976 ) especially when this PR doesn't seem to even touch any related code. And I've been unable to reproduce this locally as well.

[robertwb]

Given the uncertainty with what's going on here, another option is to put the changes in to the dataflow template itself in the short term.

[Polber]

Given the uncertainty with what's going on here, another option is to put the changes in to the dataflow template itself in the short term.

I'll draft up a PR for that