Add SigV4 request signing for AWS API authentication

[jhrozek]

This implements the requestSigner component that signs HTTP requests using AWS Signature Version 4. The signer hashes the request body, adds required headers (Authorization, X-Amz-Date, X-Amz-Security-Token), and ensures the request body remains readable after signing. A 10MB payload limit prevents memory exhaustion from oversized requests.

Related: #3567

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.98%. Comparing base (89a189d) to head (9a86774).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/auth/awssts/signer.go	78.94%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3658   +/-   ##
=======================================
  Coverage   65.97%   65.98%           
=======================================
  Files         415      416    +1     
  Lines       41121    41159   +38     
=======================================
+ Hits        27131    27157   +26     
- Misses      11897    11904    +7     
- Partials     2093     2098    +5     

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