feat(storage): implement multi stream manager for async downloads

[shubham-up-47]

This PR implements scalable multi-stream reading for Zonal Buckets by introducing a MultiStreamManager to pool and reuse gRPC connections. It mitigates performance regressions from frequent stream creation by reusing idle streams and load-balancing requests, while maintaining a proactive background connection to minimize latency during ramp-up.

[gemini-code-assist[bot]]

Summary of Changes

Hello @shubham-up-47, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the asynchronous object download capabilities of the Google Cloud Storage C++ client. It introduces a dedicated MultiStreamManager to efficiently handle multiple concurrent read streams, centralizing their lifecycle, reuse, and proactive creation. This refactoring of ObjectDescriptorImpl aims to improve performance and resource utilization for large object downloads by maintaining a dynamic pool of ready streams and intelligently managing their allocation.

Highlights

• New MultiStreamManager: Introduced a new generic MultiStreamManager template class to centralize the logic for managing multiple asynchronous streams, including their creation, reuse, and lifecycle.
• ObjectDescriptorImpl Refactoring: The ObjectDescriptorImpl has been significantly refactored to integrate and utilize the new MultiStreamManager, replacing its previous manual stream management with a more robust and scalable approach for handling concurrent read operations.
• Proactive Stream Creation and Reuse: The system now proactively creates new streams in the background to ensure a ready pool for continuous large object reads. Additionally, idle streams are intelligently identified and moved to the back of a queue for reuse, optimizing resource utilization.
• Multi-Stream Logic Activation: Previously commented-out code in object_descriptor.cc responsible for triggering subsequent stream creation when read limits are exceeded has been uncommented and activated, enabling the multi-stream behavior.
• Comprehensive Test Coverage: New unit tests have been added for the MultiStreamManager itself, and existing tests for ObjectDescriptorImpl have been updated and expanded to thoroughly validate the new multi-stream integration and behavior.

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 98.53918% with 11 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 92.96%. Comparing base (d6aff23) to head (e73e1bc). :warning: Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
...rage/internal/async/object_descriptor_impl_test.cc	97.74%	9 Missing :warning:
...d/storage/internal/async/object_descriptor_impl.cc	98.33%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15823      +/-   ##
==========================================
+ Coverage   92.95%   92.96%   +0.01%     
==========================================
  Files        2458     2460       +2     
  Lines      227589   228203     +614     
==========================================
+ Hits       211553   212153     +600     
- Misses      16036    16050      +14     

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[shubham-up-47]

/gcbrun