Prep changes for merging ValidatesRunner jobs

[reuvenlax]

In preparation for merging multiple ValidatesRunner tests into a single pipeline:

• Add a CompositePipeline class that can hold multiple pipelines. This forwards visitors and translation to all subpipelines.
• Changes to the Pipeline translation visitor to handle CompositePipeline.
• CompositePipeline expects that all steps in all pipelines have distinct names. To enable this, TransformHierarchy.java now has setRootName, which allows for a distinct root name (only the root of a hierarchy needs a distinct name. Note this is done as a method instead of a constructor parameter since we need to use it in TestPipeline, and the test name is not known when the TestPipeline rule is constructed.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.74%. Comparing base (69bdf5f) to head (3140eb7).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/java/org/apache/beam/sdk/CompositePipeline.java	88.88%	3 Missing and 1 partial ⚠️
...rg/apache/beam/sdk/runners/TransformHierarchy.java	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40436      +/-   ##
============================================
- Coverage     59.04%   58.74%   -0.30%     
+ Complexity    15624    13916    -1708     
============================================
  Files          2797     2584     -213     
  Lines        280750   271478    -9272     
  Branches      12488    11235    -1253     
============================================
- Hits         165764   159490    -6274     
+ Misses       108540   105993    -2547     
+ Partials       6446     5995     -451     

Flag	Coverage Δ
java	65.25% <90.19%> (-0.20%)	⬇️

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[github-actions]

Assigning reviewers:

R: @kennknowles for label java.

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[reuvenlax]

@kennknowles friendly ping