feat: Add S3 package pool storage backend

[mmxxixx]

Partially addresses #843

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Adds S3 as a packagePoolStorage backend, so the package pool can live in an
S3 bucket (or any S3-compatible store such as MinIO) instead of on local disk.

This implements the pool half of #843, following the suggestion in that issue.
The implementation follows azure/package_pool.go closely: same content-addressed
pool path scheme, same checksum storage flow, and it reuses the existing
S3PublishRoot config struct and the aws-sdk-go-v2 dependency already used for
S3 publishing.

• Unit tests run against the existing in-process mock S3 server (no credentials
  needed) and cover import/dedup, verify, open, remove, prefixed pools, and
  both checksum cache levels.
• PackagePoolStorage marshaling was reworked into per-type wrappers: embedding
  S3PublishRoot alongside AzureEndpoint in the previous shared wrapper would
  silently drop the colliding prefix/endpoint JSON tags (and panic in
  yaml.v3).
• Functional test S3RepoTest mirrors AzureRepoTest (t09_repo) and skips
  cleanly when AWS credentials are not present. It has been verified against
  real AWS S3, as has the full flow end-to-end (repo add, dedup, publish,
  mirror update, db cleanup, apt client install).
• New config documented in debian/aptly.conf and the man page.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (no new commands or flags)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

❌ Patch coverage is 68.23105% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.90%. Comparing base (f59b0d2) to head (397c225).

Files with missing lines	Patch %	Lines
s3/package_pool.go	69.72%	40 Missing and 26 partials ⚠️
context/context.go	0.00%	16 Missing and 1 partial ⚠️
utils/config.go	88.09%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1607      +/-   ##
==========================================
- Coverage   77.37%   76.90%   -0.48%     
==========================================
  Files         165      166       +1     
  Lines       15747    16002     +255     
==========================================
+ Hits        12185    12307     +122     
- Misses       2356     2468     +112     
- Partials     1206     1227      +21     

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