chore: Refactor CometExecRule handling of exchanges [WIP]

[andygrove]

Which issue does this PR close?

Follows on from #2844

Rationale for this change

Clean up hacky convoluted code and use the serde framework for exchanges for more consistency.

The size of CometExecRule is reduced from ~900 lines to ~500 lines with this PR.

What changes are included in this PR?

• Remove CometExecRule.operator2Proto 🎉
• CometBroadcastExchangeExec and CometShuffleExchangeExec now implement CometSink
• Move logic for determing whether shuffles are supported into CometShuffleExchangeExec
• Add requiresNativeChildren to CometOperatorSerde so we no longer need methods like convertToComet and convertToCometIfAllChildrenAreNative. It makes more sense for each operator to know whether it requires native children.

How are these changes tested?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.70817% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.40%. Comparing base (f09f8af) to head (339f744).
⚠️ Report is 737 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/execution/shuffle/CometShuffleExchangeExec.scala	73.41%	27 Missing and 19 partials ⚠️
...n/scala/org/apache/comet/rules/CometExecRule.scala	74.32%	13 Missing and 6 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #2848       +/-   ##
=============================================
- Coverage     56.12%   42.40%   -13.73%     
- Complexity      976      983        +7     
=============================================
  Files           119      167       +48     
  Lines         11743    15200     +3457     
  Branches       2251     2512      +261     
=============================================
- Hits           6591     6445      -146     
- Misses         4012     7726     +3714     
+ Partials       1140     1029      -111     

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[andygrove]

I am going to break this out into some smaller PRs