feat(cfg): add safe configuration and storage API for MRD connection pool autoscaling

[PranjalC100]

Description

This PR introduces the new configuration parameters, validation logic, and storage layer interface fields for the Multi-Range Downloader (MRD) connection pool autoscaling feature.

Specifically, it adds:

• mrd.max-connections: The maximum number of concurrent TCP connections allowed.
• mrd.min-connections: The minimum number of concurrent TCP connections allowed.
• mrd.target-pending-bytes: The target threshold for pending bytes to trigger autoscaling.
• mrd.target-pending-ranges: The target threshold for pending ranges to trigger autoscaling.

This PR is the first of a multi-part series splitting the overall connection pool autoscaling implementation. It is purely declarative and safe; because these fields are not yet wired up in the downloader wrapper or instance code, they have zero functional runtime impact on existing codepaths.

Link to the issue in case of a bug fix.

b/504489809

Testing details

1. Manual - Verified clean compilation of the entire codebase (go build ./... and basic layout validation).
2. Unit tests - Verified that all cfg package unit tests pass successfully, covering the new connection and autoscaling configuration boundaries and combinations.
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.71%. Comparing base (a64ce4a) to head (a45d385).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4739       +/-   ##
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+ Coverage        0   83.71%   +83.71%     
===========================================
  Files           0      168      +168     
  Lines           0    20752    +20752     
===========================================
+ Hits            0    17372    +17372     
- Misses          0     2734     +2734     
- Partials        0      646      +646     

Flag	Coverage Δ
unittests	83.71% <100.00%> (?)

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