feat: handle alt-svc port

[mxinden]

When alt-svc specifies a port, generate connection attempt endpoints at that port using the origin domain's resolved addresses. HTTPS record endpoints take precedence over alt-svc by virtue of ordering.

Discussed in #24.

CC @zirngibl. Note that this is a first version only, though in line with your suggestion to still use and prioritize HTTPS RR.

[Copilot]

Pull request overview

This PR implements handling of the port field in AltSvc entries. When alt-svc specifies a custom port, connection attempts are now generated at that port using the origin domain's resolved addresses. HTTPS record endpoints take precedence by virtue of ordering in the next_endpoint_to_attempt function.

Changes:

• Adds endpoint generation for alt-svc entries with custom ports (a new "alt-svc bucket" inserted between the HTTPS record bucket and the fallback bucket)
• Adds a From<HttpVersion> implementation for ConnectionAttemptHttpVersions to convert alt-svc HTTP versions
• Adds test coverage for alt-svc port scenarios (alt_svc_with_port and https_port_takes_precedence_over_alt_svc_port)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/lib.rs	Core logic: adds alt-svc port bucket in next_endpoint_to_attempt, relaxes debug_assert in connection_attempt_http_versions, and adds From<HttpVersion> for ConnectionAttemptHttpVersions
tests/network_config.rs	New test alt_svc_with_port verifying alt-svc port generates endpoints at both the custom port and the origin port
tests/https_records.rs	New test https_port_takes_precedence_over_alt_svc_port verifying HTTPS record ports take priority over alt-svc ports
tests/common/mod.rs	New out_attempt helper for constructing flexible AttemptConnection outputs with arbitrary IP, port, and HTTP version

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[KershawChang]

Looks good!