perf: Only add CopyExec if source of ScanExec is native_comet

[andygrove]

Which issue does this PR close?

Closes #1836

Rationale for this change

Avoid adding unnecessary copies. Thanks to @rluvaton for noticing this issue in #1793 (comment)

What changes are included in this PR?

Add a flag to the protobuf definition of Scan to indicate if the source of the scan reuses buffers.

How are these changes tested?

Existing tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.50%. Comparing base (f09f8af) to head (e16f02a).
Report is 220 commits behind head on main.

Files with missing lines	Patch %	Lines
.../scala/org/apache/comet/serde/QueryPlanSerde.scala	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1808      +/-   ##
============================================
+ Coverage     56.12%   58.50%   +2.37%     
- Complexity      976     1129     +153     
============================================
  Files           119      129      +10     
  Lines         11743    12582     +839     
  Branches       2251     2341      +90     
============================================
+ Hits           6591     7361     +770     
+ Misses         4012     4002      -10     
- Partials       1140     1219      +79     

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[parthchandra]

lgtm so far. (pending ci and changing from draft status)