chore: Add `scanImpl` attribute to `CometScanExec`

[andygrove]

Which issue does this PR close?

Follows on from https://github.com/apache/datafusion-comet/pull/1744

Diff between this PR and https://github.com/apache/datafusion-comet/pull/1744: https://github.com/andygrove/datafusion-comet/compare/scan-refactor-2...scan-refactor-3

Rationale for this change

This PR adds a scanImpl attribute to CometScanExec and this is populated at planning time in CometScanRule based on CometConf.COMET_NATIVE_SCAN_IMPL. All other references to CometConf.COMET_NATIVE_SCAN_IMPL have been removed from the implementation code.

This is a step toward supporting an "auto" mode in CometScanRule so that we can pick the best scan implementation and potentially have multiple scan implementations active within the same query.

What changes are included in this PR?

Add scanImpl attribute to CometScanExec and propagate this to other code.

How are these changes tested?

Existing tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 13 lines in your changes missing coverage. Please review.

Project coverage is 58.60%. Comparing base (f09f8af) to head (f6e6654). Report is 243 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/scala/org/apache/comet/rules/CometScanRule.scala	53.33%	4 Missing and 3 partials :warning:
.../scala/org/apache/comet/serde/QueryPlanSerde.scala	71.42%	0 Missing and 2 partials :warning:
...ala/org/apache/spark/sql/comet/CometScanExec.scala	75.00%	0 Missing and 2 partials :warning:
.../apache/comet/parquet/CometParquetFileFormat.scala	66.66%	0 Missing and 1 partial :warning:
...n/scala/org/apache/comet/rules/CometExecRule.scala	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1746      +/-   ##
============================================
+ Coverage     56.12%   58.60%   +2.47%     
- Complexity      976     1131     +155     
============================================
  Files           119      130      +11     
  Lines         11743    12663     +920     
  Branches       2251     2362     +111     
============================================
+ Hits           6591     7421     +830     
- Misses         4012     4058      +46     
- Partials       1140     1184      +44     

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