[prism] Add fusion.

[lostluck]

Add robust fusion heuristics to Prism's graph Pre-processing, leading to improved performance when it's enabled. In this PR it's enabled by default, but in a subsequent PR, it'll be enabled by default in the stand alone command, and generally all variants except the "test" variant.

• Largely cribbed from Python fn_api_runner/translations.py, which is based on two main heuristics: Avoiding side input conflicts, and indicating situations where a producing transform must not be fused with a consuming transform.
• Updated handlers and fact computation to use return structs.
• Add simple benchmarks for the Go Direct Runner, and Prism to demonstrate fusion was effective.
• Clean up no-longer necessary exec side override for environments (handled in handlers now).
• Fix Bug in ProcessBundle handling where a stage without data outputs would not send progress or split requests. This broke the channel splitting test in particular.
  • As a bonus, this moves that async processing into the progress handling loop, cleaning up that handling.

Minor fixes:

• Fix Transform UniqueName for Combine extract output.
• Fix channel splitting separation harness test to have an explicit Reshuffle, as it can't be automatically added for that synthetic test.
  • Reduce wait time for separation harness polling.

Note: This can be improved if we add "Flatten Unnzipping/Sinking" where Flattens can be removed from the graph, and executed implicitly. It's a more elaborate graph transformation however, since it involves duplicating downstream transforms, and allowing PCollections to have multiple producers. For now, we ignore SDK side flattens (which are only sometimes permittable), and force Runner side flattens to not be fused, since runner side transforms can't fuse at this time.

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[codecov]

Codecov Report

Merging #29306 (95e3f11) into master (87ca614) will increase coverage by 0.01%.
Report is 7 commits behind head on master.
The diff coverage is 90.61%.

❗ Current head 95e3f11 differs from pull request most recent head 39939ce. Consider uploading reports for the commit 39939ce to get more accurate results

@@            Coverage Diff             @@
##           master   #29306      +/-   ##
==========================================
+ Coverage   38.30%   38.32%   +0.01%     
==========================================
  Files         690      690              
  Lines      102048   102123      +75     
==========================================
+ Hits        39088    39135      +47     
- Misses      61378    61402      +24     
- Partials     1582     1586       +4     

Flag	Coverage Δ
go	53.48% <90.61%> (+0.01%)	⬆️

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

Files	Coverage Δ
sdks/go/pkg/beam/runners/prism/internal/execute.go	86.95% <ø> (-0.28%)	⬇️
.../go/pkg/beam/runners/prism/internal/handlepardo.go	95.89% <100.00%> (+0.23%)	⬆️
...go/pkg/beam/runners/prism/internal/handlerunner.go	82.19% <93.33%> (-1.81%)	⬇️
...s/go/pkg/beam/runners/prism/internal/preprocess.go	94.44% <95.74%> (-0.23%)	⬇️
...o/pkg/beam/runners/prism/internal/handlecombine.go	87.39% <46.15%> (-4.43%)	⬇️
sdks/go/pkg/beam/runners/prism/internal/stage.go	82.90% <74.07%> (-0.08%)	⬇️

... and 4 files with indirect coverage changes

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[riteshghorse]

LGTM! Thank you for the detailed description!