new feature: support content_language in opendal::metadata and opendal:capability

### Feature Description

support content_language in opendal::options::WriteOptions

### Problem and Solution

comment on https://github.com/apache/opendal/pull/6468#discussion_r2249120305
not support content_language in opendal::options::WriteOptions

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [x] Yes, I am willing to contribute to the development of this feature.

Start at opendal::options::WriteOptions and trace the related metadata and capability entry points; review the linked pull request discussion for the expected content_language behavior. Done means content_language is supported consistently in WriteOptions, metadata, and capability as described by the issue.

Written by the indexing model from the issue text.