[#23893] Support composite scope transform metadata

[lostluck]

• Adds PTransform annotation plumbing through context.Context
  • Lays groundwork to do the same for PTransform Display Data.
• Deferred to another PR: Doing the same for environments.
  • Punted from this PR since it's a much broader change to install.

Working on #23893.

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[codecov]

Codecov Report

Merging #29204 (fdb601f) into master (2f8eebb) will increase coverage by 0.01%.
Report is 61 commits behind head on master.
The diff coverage is 68.96%.

@@            Coverage Diff             @@
##           master   #29204      +/-   ##
==========================================
+ Coverage   38.36%   38.38%   +0.01%     
==========================================
  Files         687      688       +1     
  Lines      101747   101804      +57     
==========================================
+ Hits        39039    39078      +39     
- Misses      61129    61143      +14     
- Partials     1579     1583       +4     

Flag	Coverage Δ
go	53.45% <68.96%> (+0.02%)	⬆️

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

Files	Coverage Δ
sdks/go/pkg/beam/core/graph/graph.go	60.65% <100.00%> (ø)
sdks/go/pkg/beam/core/graph/scope.go	0.00% <ø> (ø)
sdks/go/pkg/beam/core/runtime/graphx/translate.go	37.61% <100.00%> (+0.48%)	⬆️
sdks/go/pkg/beam/pipeline.go	45.83% <0.00%> (-9.17%)	⬇️
.../go/pkg/beam/core/runtime/contextreg/contextreg.go	68.18% <68.18%> (ø)

... and 2 files with indirect coverage changes

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[riteshghorse]

Looks good, thank you!

[lostluck]

Thanks for the review!