feat(services/oss): support content_encoding in write operations

[ITpandaffm]

Which issue does this PR close?

Closes #7656.

Rationale for this change

The OSS backend currently does not honor OpWrite::content_encoding(), even
though the write option already exists and other object storage backends such as
S3 and GCS support it.

Users who upload gzip/br/deflate objects to Aliyun OSS through OpenDAL cannot
preserve the Content-Encoding metadata. This breaks CDN/browser transparent
decompression for compressed static objects.

What changes are included in this PR?

• core/services/oss/src/backend.rs
  • Set write_with_content_encoding: true in the OSS capability table.
• core/services/oss/src/core.rs
  • Import http::header::CONTENT_ENCODING.
  • Forward args.content_encoding() in shared write metadata headers, covering
    PutObject and AppendObject.
  • Add a content_encoding parameter to oss_initiate_upload and set the
    header on InitiateMultipartUpload.
• core/services/oss/src/writer.rs
  • Pass self.op.content_encoding() into oss_initiate_upload for multipart
    writes.

Are there any user-facing changes?

Yes. OSS now advertises write_with_content_encoding, and
op.write_with(...).content_encoding(...) will preserve Content-Encoding
metadata on written objects.

Existing users who do not set content_encoding are unaffected.

Tests

• cargo fmt --check
• cargo check -p opendal --features services-oss --no-default-features
• cargo clippy -p opendal --features services-oss --no-default-features -- -D warnings
• cargo test -p opendal-service-oss --no-default-features

[Xuanwo]

Thank you!