Add connectHeaders to Proxy for HTTP CONNECT tunneling

[o-nnerb]

Summary

• Adds connectHeaders to Proxy, exposing async-http-client's connectHeaders (add proxy-header support swift-server/async-http-client#912, available since the pinned 1.36.0) — extra headers sent only on the HTTP CONNECT request used to establish the proxy tunnel.
• connectHeaders is composed as a Property builder, the same way Form scopes its own per-part headers: Proxy becomes generic over Headers: Property, and a new init resolves that subtree via _makeProperty + search(for: HeaderNode.self) into a local HTTPHeaders value, entirely separate from the request's own headers.
• Internals.Proxy gains a connectHeaders field, threaded only through the .http branch of build(). Excluded from Hashable — NIOHTTP1.HTTPHeaders isn't Hashable — mirroring how async-http-client's own Proxy type handles the same constraint.
• The two existing initializers (authorization: and connection:) are unchanged, constrained to Headers == EmptyProperty.

DataTask {
    BaseURL("example.com")
    Proxy(host: "proxy.example.com", port: 8080) {
        CustomHeader(name: "X-Proxy-Token", value: "abc123")
    }
}

Marked breaking-changes because Proxy is now generic (Proxy<Headers: Property>) — source-compatible for every existing call site, but any code that spells the type explicitly (e.g. let p: Proxy) needs Proxy<EmptyProperty>.

Implements #279.

Test plan

• swift test --filter RequestDLTests.ProxyTests
• swift test --filter RequestDLInternalsTests.InternalsProxyTests
• swift test (full suite, 1103 tests)
• swift format lint --recursive --strict Sources Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (3c2f31d) to head (59c7496).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
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  Coverage   98.24%   98.24%           
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  Files           3        3           
  Lines         857      857           
=======================================
  Hits          842      842           
  Misses         15       15           

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