Add set state in Go

[damccorm]

This continues to grow our state support by adding map state support. Once this is 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/96/files#diff-46f0bd8f9f6b541e840079960d684ba78e6513f1293b673d08faaec94d48a234 - and it produced output like:

swimming: false. Total num keys: 2. With keys: [swimming36 swimming]
hourly: false. Total num keys: 2. With keys: [hourly52 hourly]
hourly: true. Total num keys: 3. With keys: [hourly33 hourly52 hourly]
hourly: true. Total num keys: 4. With keys: [hourly52 hourly hourly1 hourly33]
About: false. Total num keys: 2. With keys: [About About38]
advise: false. Total num keys: 2. With keys: [advise18 advise]
advise: true. Total num keys: 3. With keys: [advise18 advise68 advise]
advise: true. Total num keys: 4. With keys: [advise68 advise18 advise4 advise]
lawful: false. Total num keys: 2. With keys: [lawful9 lawful]
lawful: true. Total num keys: 3. With keys: [lawful9 lawful lawful49]
lawful: true. Total num keys: 4. With keys: [lawful49 lawful38 lawful lawful9]

Note that this doesn't add support for Remove (to remove a single key). This will come in a future pr and has been noted in #22736

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 #22919 (260ec30) into master (0c82583) will decrease coverage by 0.00%.
The diff coverage is 53.17%.

@@            Coverage Diff             @@
##           master   #22919      +/-   ##
==========================================
- Coverage   73.69%   73.68%   -0.01%     
==========================================
  Files         713      713              
  Lines       94821    94930     +109     
==========================================
+ Hits        69874    69945      +71     
- Misses      23648    23684      +36     
- Partials     1299     1301       +2     

Flag	Coverage Δ
go	51.00% <53.17%> (+0.04%)	⬆️

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%> (ø)
sdks/go/pkg/beam/core/runtime/exec/translate.go	12.72% <0.00%> (-0.22%)	⬇️
sdks/go/pkg/beam/core/runtime/graphx/translate.go	38.42% <0.00%> (-0.50%)	⬇️
sdks/go/pkg/beam/pardo.go	41.79% <0.00%> (-0.32%)	⬇️
sdks/go/pkg/beam/core/state/state.go	76.57% <80.72%> (+1.29%)	⬆️
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
sdks/python/apache_beam/runners/direct/executor.py	96.46% <0.00%> (-0.55%)	⬇️
sdks/python/apache_beam/transforms/util.py	96.22% <0.00%> (+0.15%)	⬆️
...ks/python/apache_beam/runners/worker/sdk_worker.py	89.09% <0.00%> (+0.15%)	⬆️
... and 6 more

📣 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