feat(gax): add BidiStreamOptions for bidirectional streaming

[suzmue]

Add BidiStreamOptions to google_cloud_gax::options to support configuring bidirectional streaming RPC parameters such as request channel buffer capacity. This is the only special option we currently have and I do not have other per stream options that I anticipate we set, so I decided to not implement a trait similar to RequestBuilder.

BidiStreamOptions holds a base RequestOptions and request_channel_capacity defaulting to 16, bounded between 1 and MAX_REQUEST_CHANNEL_CAPACITY.

We also provide From conversions between RequestOptions and BidiStreamOptions with user-focused documentation. This enables our users to pass either into the with_options configuration.

For #2318

[gemini-code-assist]

Code Review

This pull request introduces BidiStreamOptions to configure bidirectional streaming RPCs, wrapping standard RequestOptions and adding settings for internal request channel capacity. The feedback suggests defining a public constant DEFAULT_REQUEST_CHANNEL_CAPACITY to avoid magic numbers, and adding fluent builder methods (with_request_channel_capacity and with_request_options) that consume and return self to align with the repository's style guide.

[suzmue]

Corresponding template changes: googleapis/librarian#7246

Generated code using this: https://github.com/googleapis/google-cloud-rust/compare/main...suzmue:google-cloud-rust:bidi-stream-options-showcase?expand=1

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.26%. Comparing base (2a3e1e0) to head (25def79).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax/src/options.rs	88.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6341      +/-   ##
==========================================
- Coverage   96.26%   96.26%   -0.01%     
==========================================
  Files         283      283              
  Lines       73110    73160      +50     
==========================================
+ Hits        70380    70428      +48     
- Misses       2730     2732       +2     

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