This Pull Request addresses the maintainer's feedback regarding the addition of the ZstdCompressor to the S3 output plugin.

Changes Made:

1. Removed Duplicate ZstdCompressor Class:

   • Deleted lib/fluent/plugin/s3_compressor_zstd.rb as the ZstdCompressor class was already defined in out_s3.rb, eliminating duplication.

2. Added Tests for ZstdCompressor in test_out_s3.rb:

   • Moved the tests related to ZstdCompressor from test_in_s3.rb to test_out_s3.rb since they pertain to the output plugin.
   • Added test cases test_configure_with_mime_type_zstd and test_write_with_zstd to verify the proper functioning of ZstdCompressor.

3. Ensured ZstdCompressor is Properly Registered:

   • Verified that ZstdCompressor is correctly registered in the COMPRESSOR_REGISTRY within out_s3.rb.

4. Fixed Test Failures:

   • Adjusted the s3path in test_write_with_zstd to match the expected file extension for zstd compression, resolving the test failure.

5. Resolved Gem Build Error:

   • Removed s3_compressor_zstd.rb from the Git index to fix the gem build error caused by the file being listed but not present.

Notes:

• All changes have been made according to the maintainer's comments.
• Tests have been updated and all pass successfully.
• Documentation and code comments have been updated accordingly.

Please review the changes, and let me know if any further adjustments are needed.