feat(services/oss): Rename allow_anonymous to skip_signature

[YuangGao]

Which issue does this PR close?

• Part of new feature: rename allow_anonymous to skip_signature #7537.

Rationale for this change

This PR renames allow_anonymous to skip_signature for OSS, matching the naming used by object_store and aligning with the S3 rename in #7544.

What changes are included in this PR?

• Add OssBuilder::skip_signature and OssConfig::skip_signature.
• Deprecate OssBuilder::allow_anonymous and OssConfig::allow_anonymous;
  both keep working and are merged into skip_signature at build time.
• Update .NET / Java / Python bindings accordingly (old field kept, new
  field added, old marked deprecated).
• Update docs.md to reference the new option.

Are there any user-facing changes?

No breaking changes. allow_anonymous still works but emits a deprecation warning suggesting skip_signature.