feat(services/gcs): 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 the option to skip_signature and changes the semantics to unconditional skip

What changes are included in this PR?

• Add GcsBuilder::skip_signature and GcsConfig::skip_signature.
• Deprecate GcsBuilder::allow_anonymous and GcsConfig::allow_anonymous; both keep working and are merged into skip_signature at build time.
• Change signing semantics from fallback-on-error to eager skip, matching other services.
• Update .NET / Java / Python bindings accordingly

Are there any user-facing changes?

Behavior change for users who relied on the fallback semantics: if you previously set allow_anonymous(true) and provided valid credentials, GCS would sign requests. Now skip_signature(true) (and the deprecated
allow_anonymous(true)) unconditionally skips signing. Users who want signing should not set this option.

No source-breaking changes. allow_anonymous still compiles with a deprecation warning suggesting skip_signature.

[Xuanwo]

Thank you!