[awaisdev5765]

Motivation

Fixes #151 — clients currently cannot read the headers a server sends in the 101 Switching Protocols upgrade response (e.g. an auth token minted server-side during the upgrade flow).

Changes

• New connect overloads whose onUpgradeWithResponse closure receives the HTTPResponseHead alongside the WebSocket:
  • WebSocket.connect(to:scheme:...)
  • WebSocketClient.connect(...) (incl. proxy variants)
  • async/await variants
• Existing onUpgrade APIs are unchanged — they forward through to the new variants and discard the head, so this is fully source-compatible.
• 2 new tests covering the Future and async flows (testUpgradeResponseHeadersAreReceived, testUpgradeResponseHeadersAreReceivedAsync).

Result

Full suite passes: 27 tests, 0 failures. A client can now do:

WebSocket.connect(to: url, headers: ["Auth": token], on: group) { ws, responseHead in
    let serverToken = responseHead.headers.first(name: "X-Auth-Token")
    // ...
}