[BEAM-14347] Add generic registration for Combiners

[damccorm]

This is a continuation of the work started in #17429 and #17574 and extends the existing registration logic to allow registration of combiners.

Design doc: https://docs.google.com/document/d/1MLZLUhb-Sn7AJ666yn3TnYgMKE6wiXIBs7cjB_20hMA/edit?usp=sharing

After this, the big remaining chunks of work are:

1. Perf testing
2. Updates to the docs/our examples

---

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).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• 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.

[asf-ci]

Can one of the admins verify this patch?

[asf-ci]

Can one of the admins verify this patch?

[asf-ci]

Can one of the admins verify this patch?

[asf-ci]

Can one of the admins verify this patch?

[codecov]

Codecov Report

Merging #17579 (b2419ad) into master (85d9824) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17579      +/-   ##
==========================================
- Coverage   73.95%   73.95%   -0.01%     
==========================================
  Files         693      693              
  Lines       91716    91690      -26     
==========================================
- Hits        67832    67812      -20     
+ Misses      22635    22629       -6     
  Partials     1249     1249              

Flag	Coverage Δ
go	50.37% <ø> (ø)

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

Impacted Files	Coverage Δ
sdks/python/apache_beam/utils/interactive_utils.py	95.12% <0.00%> (-2.44%)	⬇️
.../python/apache_beam/io/gcp/tests/pubsub_matcher.py	90.47% <0.00%> (-1.11%)	⬇️
sdks/python/apache_beam/internal/metrics/metric.py	93.00% <0.00%> (-1.00%)	⬇️
...ks/python/apache_beam/runners/worker/sdk_worker.py	88.94% <0.00%> (-0.16%)	⬇️
sdks/python/apache_beam/io/gcp/gcsio.py	92.26% <0.00%> (ø)
sdks/python/apache_beam/io/gcp/pubsub.py	90.65% <0.00%> (ø)
sdks/python/apache_beam/io/gcp/bigquery.py	63.87% <0.00%> (ø)
sdks/python/apache_beam/io/gcp/gcsfilesystem.py	88.23% <0.00%> (ø)
.../apache_beam/io/gcp/datastore/v1new/datastoreio.py	86.16% <0.00%> (ø)
...beam/transforms/fully_qualified_named_transform.py	100.00% <0.00%> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 85d9824...b2419ad. Read the comment docs.

[damccorm]

R: @lostluck

Since this one has a bunch of generic + generated code and is a tougher one to review without context, I'm going to send this directly to you

[github-actions]

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

[lostluck]

Naming aside: LGTM.

[lostluck]

Will merge once the conflict from the emitters/iters is resolved.

[damccorm]

Will merge once the conflict from the emitters/iters is resolved.

Done