Add DataSource/FileSource proto hooks and FileScanConfig serde

[kumarUjjawal]

Which issue does this PR close?

• Closes Proto: add DataSource/FileSource try_to_proto hook + port FileScanConfig serde #23497.

Rationale for this change

File scan serialization currently depends on central type downcasts in datafusion-proto.

This PR adds the foundation needed to move each data source to its own protobuf hooks while preserving the existing wire format and fallback behavior.

It unblocks the concrete source migrations tracked by #23516, #23517, and #23518.

What changes are included in this PR?

• Add feature-gated try_to_proto hooks to DataSource and FileSource.
• Delegate serialization from DataSourceExec through FileScanConfig to its concrete FileSource.
• Add shared FileScanConfig protobuf encoding and decoding in the datasource crate.
• Keep the existing central serializer as the fallback for sources that still return Ok(None).
• Preserve the legacy protobuf wire format.
• Add compatibility test

Are these changes tested?

Yes

Are there any user-facing changes?

This adds non-breaking, feature-gated public serialization hooks for data source and file source implementations.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.23773% with 122 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.69%. Comparing base (1734d4b) to head (bc9a8e5).

Files with missing lines	Patch %	Lines
...atafusion/datasource/src/file_scan_config/proto.rs	86.46%	10 Missing and 39 partials ⚠️
datafusion/proto/src/physical_plan/mod.rs	88.54%	20 Missing and 17 partials ⚠️
datafusion/datasource/src/file_scan_config/mod.rs	48.57%	35 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23683      +/-   ##
==========================================
+ Coverage   80.67%   80.69%   +0.01%     
==========================================
  Files        1087     1088       +1     
  Lines      367541   368315     +774     
  Branches   367541   368315     +774     
==========================================
+ Hits       296529   297200     +671     
- Misses      53355    53400      +45     
- Partials    17657    17715      +58     

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