Add bag state support

[damccorm]

This continues to grow our state support by adding bag state support. Once both this and #22815 are in, I'll add a follow up PR with an integration test. In the meantime, I tested manually by running the hacked wordcount found here - https://github.com/damccorm/beam/pull/86/files#diff-46f0bd8f9f6b541e840079960d684ba78e6513f1293b673d08faaec94d48a234 - and it produced output like:

...
away([]): 1
away([I]): 1
away([I I]): 1
away([I I I]): 1
away([I I I I]): 1
away([I I I I I]): 1
away([I I I I I I]): 1
away([I I I I I I I]): 1
away([I I I I I I I I]): 1
away([I I I I I I I I I]): 1
away([I I I I I I I I I I]): 1
away([I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I I I I I]): 1
away([I I I I I I I I I I I I I I I I I I I I I I I I I I I I I]): 1
injuries([]): 1
injuries([I]): 1
injuries([I I]): 1
necessities([]): 1
Be([]): 1
Be([I]): 1
Be([I I]): 1
Be([I I I]): 1
Be([I I I I]): 1
Be([I I I I I]): 1
Be([I I I I I I]): 1
Be([I I I I I I I]): 1
Be([I I I I I I I I]): 1
Be([I I I I I I I I I]): 1
Be([I I I I I I I I I I]): 1
Be([I I I I I I I I I I I]): 1
Be([I I I I I I I I I I I I]): 1
Be([I I I I I I I I I I I I I]): 1
Be([I I I I I I I I I I I I I I]): 1
Be([I I I I I I I I I I I I I I I]): 1
...

Part of #22736

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.

[codecov]

Codecov Report

Merging #22816 (926981c) into master (8d886a9) will decrease coverage by 0.04%.
The diff coverage is 27.50%.

@@            Coverage Diff             @@
##           master   #22816      +/-   ##
==========================================
- Coverage   73.97%   73.92%   -0.05%     
==========================================
  Files         713      713              
  Lines       94048    94151     +103     
==========================================
+ Hits        69571    69603      +32     
- Misses      23191    23262      +71     
  Partials     1286     1286              

Flag	Coverage Δ
go	51.24% <27.50%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/graph/fn.go	84.54% <0.00%> (-0.02%)	⬇️
sdks/go/pkg/beam/core/runtime/exec/translate.go	13.55% <0.00%> (-0.13%)	⬇️
sdks/go/pkg/beam/core/runtime/graphx/translate.go	41.06% <0.00%> (-0.53%)	⬇️
sdks/go/pkg/beam/core/runtime/exec/userstate.go	12.82% <2.27%> (-4.15%)	⬇️
sdks/go/pkg/beam/core/state/state.go	75.30% <76.19%> (+0.94%)	⬆️

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

[damccorm]

R: @riteshghorse

[github-actions]

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

[damccorm]

Run GoPortable PreCommit

[riteshghorse]

LGTM