feat(binding/go): expose all metata fields to golang

[dentiny]

Rationale for this change

Language binding implementation should be compatible feature-wise.

What changes are included in this PR?

This PR exposes all metadata fields to C and golang.

Are there any user-facing changes?

No.

AI Usage Statement

GPT-5.5 helped me make the change, opus-4.8 and manual review

[yuchanns]

Can we add some metadata-focused cases in TestBehavior?

FYI:

• https://github.com/apache/opendal/blob/main/core/tests/behavior/async_write.rs#L133-L247
  (write metadata fields: cache_control, content_type, content_disposition, content_encoding, user_metadata, plus write-returned metadata checks)
• https://github.com/apache/opendal/blob/main/core/tests/behavior/async_stat.rs#L556-L620
  (versioned stat behavior)
• https://github.com/apache/opendal/blob/main/core/tests/behavior/async_read.rs#L952-L1012
  (versioned read behavior)
• https://github.com/apache/opendal/blob/main/core/tests/behavior/async_delete.rs#L239-L304
  (versioned delete behavior)
• https://github.com/apache/opendal/blob/main/core/tests/behavior/async_list.rs#L654-L710
  (list_with_versions, list_with_deleted, and metadata().is_deleted)

[dentiny]

Can we add some metadata-focused cases in TestBehavior?

Yeah that makes sense

• I added some behavior tests for stats
• We still miss a few APIs on read and write side, for example, write/read with version; I plan to add more tests along the way (e.g., when we add write with version, we use this PR to check version returned).

@FrankYang0529 and I are working together to fill the gaps between go binding and rust core APIs.

[yuchanns]

LGTM

[dentiny]

I plan to merge this PR now so @FrankYang0529 could be unblocked on #7645.
I will be responsible if any issues.