[WIP] Add AS-aware relay selection

[Mshehu5]

This PR is a proof of concept for AS-aware, stateless relay selection in payjoin-cli and is meant to move the discussion in #919 forward. Reviewers will probably want to read #919 for the full context behind the design and tradeoffs.

The implementation adds optional ASMap-based filtering for trusted directories and OHTTP relays then demonstrates relay selection that separates POST and POLL traffic without storing a selected relay or relay index. Relay ordering is derived from the receiver key, request/message type and short time windows with polling avoiding POST-reserved AS buckets for nearby windows.

I also added comments around parts of the implementation that may be candidates for moving into the payjoin crate or possibly into a reusable external crate. These comments are suggestions not final API proposals and reviewer feedback would be very welcome. One reason for calling these pieces out is the concern raised in #919: asking every downstream wallet to reimplement this behavior could create extra work and consistency problems.

Disclosure: co-authored by Codex

Pull Request Checklist

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[coveralls]

Coverage Report for CI Build 26173671454

Coverage decreased (-0.9%) to 84.348%

Details

• Coverage decreased (-0.9%) from the base build.
• Patch coverage: 263 uncovered changes across 5 files (551 of 814 lines covered, 67.69%).
• 7 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
payjoin-cli/src/app/v2/relay_selection.rs	499	374	74.95%
payjoin-cli/src/app/v2/ohttp.rs	76	3	3.95%
payjoin-cli/src/app/config.rs	88	55	62.5%
payjoin-cli/src/app/v2/mod.rs	142	111	78.17%
payjoin-cli/src/app/mod.rs	9	8	88.89%

Coverage Regressions

7 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-cli/src/app/v2/ohttp.rs	6	9.2%
payjoin-cli/src/app/config.rs	1	73.41%

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Coverage Stats

Relevant Lines:	14330
Covered Lines:	12087
Line Coverage:	84.35%
Coverage Strength:	378.93 hits per line

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💛 - Coveralls

[arminsabouri]

cc @nothingmuch to approach Ack/Nack