[YAML] Schema-producing Create.

[robertwb]

Yaml's Create will now always create schema'd data. For non-row elements, this will be a single Row with an element field.

This brings increased consistency to the YAML API.

---

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.

[codecov]

Codecov Report

Merging #29030 (62f2576) into master (e7a6405) will decrease coverage by 0.03%.
Report is 25 commits behind head on master.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #29030      +/-   ##
==========================================
- Coverage   38.39%   38.36%   -0.03%     
==========================================
  Files         686      686              
  Lines      101651   101673      +22     
==========================================
- Hits        39025    39009      -16     
- Misses      61042    61089      +47     
+ Partials     1584     1575       -9     

Flag	Coverage Δ
python	29.98% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
sdks/python/apache_beam/yaml/yaml_provider.py	0.00% <0.00%> (ø)

... and 10 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @liferoad for label python.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[liferoad]

LGTM. Not sure why some pre-commits keep failing.