Merge bitcoin/bitcoin#28649: Do the SOCKS5 handshake reliably

af0fca530e netbase: use reliable send() during SOCKS5 handshake (Vasil Dimov)
1b19d1117c sock: change Sock::SendComplete() to take Span (Vasil Dimov)

Pull request description:

  The `Socks5()` function which does the SOCKS5 handshake with the SOCKS5 proxy sends bytes to the socket without retrying partial writes.

  `send(2)` may write only part of the provided data and return. In this case the caller is responsible for retrying the operation with the remaining data. Change `Socks5()` to do that. There is already a method `Sock::SendComplete()` which does exactly that, so use it in `Socks5()`.

  A minor complication for this PR is that `Sock::SendComplete()` takes `std::string` argument whereas `Socks5()` has `std::vector<uint8_t>`. Thus the necessity for the first commit. It is possible to do also in other ways - convert the data in `Socks5()` to `std::string` or have just one `Sock::SendComplete()` that takes `void*` and change the callers to pass `str.data(), str.size()` or `vec.data(), vec.size()`.

  This came up while testing https://github.com/bitcoin/bitcoin/pull/27375.

ACKs for top commit:
  achow101:
    ACK af0fca530e
  jonatack:
    ACK af0fca530e
  pinheadmz:
    ACK af0fca530e

Tree-SHA512: 1d4a53d0628f7607378038ac56dc3b8624ce9322b034c9547a0c3ce052eafb4b18213f258aa3b57bcb4d990a5e0548a37ec70af2bd55f6e8e6399936f1ce047a

src/test/fuzz/socks5.cpp

@@ -32,7 +32,9 @@ FUZZ_TARGET(socks5, .init = initialize_socks5)
     ProxyCredentials proxy_credentials;
     proxy_credentials.username = fuzzed_data_provider.ConsumeRandomLengthString(512);
     proxy_credentials.password = fuzzed_data_provider.ConsumeRandomLengthString(512);
-    InterruptSocks5(fuzzed_data_provider.ConsumeBool());
+    if (fuzzed_data_provider.ConsumeBool()) {
+        g_socks5_interrupt();
+    }
     // Set FUZZED_SOCKET_FAKE_LATENCY=1 to exercise recv timeout code paths. This
     // will slow down fuzzing.
     g_socks5_recv_timeout = (fuzzed_data_provider.ConsumeBool() && std::getenv("FUZZED_SOCKET_FAKE_LATENCY") != nullptr) ? 1ms : default_socks5_recv_timeout;