feat(services/upyun): Add user_metadata support

[userzhy]

Which issue does this PR close?

Part of #4842 (partially, upyun part).

Rationale for this change

This PR implements user-defined metadata support for the upyun service, as tracked in issue #4842. User metadata allows users to store custom key-value pairs along with their files, which is useful for various use cases like storing file metadata such as tenant, catalog, database, table, etc.

What changes are included in this PR?

• Added X_UPYUN_META_PREFIX constant (x-upyun-meta-) in core.rs
• Updated upload method to set user metadata headers when writing files
• Updated initiate_multipart_upload method to set user metadata headers for multipart uploads
• Added write_with_user_metadata: true capability in backend.rs
• Updated stat method to parse and return user metadata using parse_prefixed_headers

Are there any user-facing changes?

Yes. Users can now use write_with_user_metadata to set custom metadata when uploading files to upyun, and retrieve the metadata via stat operation.

AI Usage Statement

This PR was developed with the assistance of GitHub Copilot (Claude Opus 4.5).