[Enhancement](file-cache) Add fine-grained control for compaction file cache

[gavinchou]

Support selective caching of index files only during compaction in cloud mode.

Cherry-picked from #60609

Changes:

• Add two mBool configs to control index-only caching for base and cumulative compaction:

  • enable_file_cache_write_base_compaction_index_only (default: false)
  • enable_file_cache_write_cumu_compaction_index_only (default: false)

• Extend RowsetWriterContext with compaction_output_write_index_only field
  to mark whether only index files should be cached

• Modify get_file_writer_options() to accept is_index_file parameter:

  • When compaction_output_write_index_only=true and is_index_file=false,
    set write_file_cache=false to skip caching data files
  • Index files continue to be cached normally

• Update file writer creation call sites to pass is_index_file parameter:

  • Index file writers: pass true
  • Segment (data) file writers: pass false

Benefits:

• Reduces cache pressure by avoiding caching large data files during compaction
• Preserves index file caching for query performance
• Provides separate control for base and cumulative compaction strategies
• Maintains backward compatibility with default settings

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[gavinchou]

run buildall

[gavinchou]

run buildall

[doris-robot]

BE UT Coverage Report

Increment line coverage 87.50% (28/32) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.98% (19183/36205)
Line Coverage	36.17% (178753/494252)
Region Coverage	32.79% (138548/422522)
Branch Coverage	33.78% (60193/178191)