minor: Move operator2Proto to CometExecRule

[andygrove]

Which issue does this PR close?

Part of #2757

Rationale for this change

This is a small step in a larger refactoring.

operator2Proto is only called from CometExecRule, so this PR moves it to that class and makes it a private method.

This moves all the code for determining whether native operators are enabled and supported into a single file.

What changes are included in this PR?

There are no functional changes in this PR. It just moves some code around.

The next PR after this (#2768) is a little more interesting.

How are these changes tested?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.07874% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.84%. Comparing base (f09f8af) to head (0d3d72c).
⚠️ Report is 692 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/scala/org/apache/comet/rules/CometExecRule.scala	70.07%	22 Missing and 16 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2767      +/-   ##
============================================
+ Coverage     56.12%   58.84%   +2.71%     
- Complexity      976     1501     +525     
============================================
  Files           119      164      +45     
  Lines         11743    14014    +2271     
  Branches       2251     2371     +120     
============================================
+ Hits           6591     8246    +1655     
- Misses         4012     4567     +555     
- Partials       1140     1201      +61     

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[mbutrovich]

Looks like an improvement to me, thanks @andygrove!