feat(storage): add feature flag and models for appendable upload

[vsharonlynn]

Introduces the google_cloud_unstable_storage_appendable_upload feature flag to gate the upcoming GCS Appendable Upload functionality.

[gemini-code-assist]

Code Review

This pull request introduces the initial infrastructure for appendable object uploads in the storage crate, gated by a new unstable configuration flag. The changes include updates to build triggers, lint configurations, and the addition of new data structures and modules. Feedback focuses on maintaining SDK consistency by using ergonomic model types instead of raw proto types, applying #[allow(dead_code)] to structs used only in mocks to prevent compiler warnings, and restricting module visibility to pub(crate) to align with existing internal modules.

[gemini-code-assist]

The Takeover variant uses the raw proto type crate::google::storage::v2::AppendObjectSpec, whereas the Create variant uses the ergonomic model type crate::model::WriteObjectSpec. To maintain consistency and follow the SDK's pattern of abstracting internal proto types, AppendObjectSpec should be modeled in crate::model and used here instead of the raw proto type. Note that a feature does not have to be fully implemented in a single pull request if the remaining work is documented in the PR description or as a TODO comment in the code.

References

1. Maintain patterns consistent with existing crates like auth and gax. In this SDK, types in model_ext.rs should generally use or wrap types from crate::model rather than exposing raw proto types directly.
2. A feature does not have to be fully implemented in a single pull request if the remaining work is documented in the PR description or as a TODO comment in the code, allowing for continuous improvement.

[gemini-code-assist]

This struct is documented as being "only used in mocks", similar to WriteObjectRequest. It should include #[allow(dead_code)] to avoid compiler warnings in production builds where the mocks are not active.

#[non_exhaustive]
#[allow(dead_code)]
pub struct AppendObjectRequest {

[gemini-code-assist]

The append_object module is declared as pub, but most other internal modules in this file (e.g., write_object, read_object, bidi) are pub(crate). Unless this module is intended to be part of the public API surface (which is typically exposed through the Storage client or builders), it should be pub(crate) for consistency and better encapsulation.

Suggested change

	pub mod append_object;
	pub(crate) mod append_object;

References

1. Follow local standards: Maintain patterns consistent with existing code. Most internal logic modules in this crate use pub(crate) visibility. ^(link)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (e199cf6) to head (3deeb71).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5631      +/-   ##
==========================================
- Coverage   97.76%   97.76%   -0.01%     
==========================================
  Files         221      221              
  Lines       51115    51115              
==========================================
- Hits        49973    49972       -1     
- Misses       1142     1143       +1     

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