Merge bitcoin/bitcoin#28287: rpc, test: add `sendmsgtopeer` rpc and a test for net-level deadlock situation

b3a93b409e7fb33af77bd34a269a3eae71d3ba83 test: add functional test for deadlock situation (Martin Zumsande)
3557aa4d0ab59c18807661a49070b0e5cbfecde3 test: add basic tests for sendmsgtopeer to rpc_net.py (Martin Zumsande)
a9a1d69391596f57851f545fae12f8851149d2c3 rpc: add test-only sendmsgtopeer rpc (Martin Zumsande)

Pull request description:

  This adds a `sendmsgtopeer` rpc (for testing only) that allows a node to send a message (provided in hex) to a peer.
  While we would usually use a `p2p` object instead of a node for this in the test framework, that isn't possible in situations where this message needs to trigger an actual interaction of multiple nodes.

  Use this rpc to add test coverage for the bug fixed in #27981 (that just got merged):
  The test lets two nodes (almost) simultaneously send a single large (4MB) p2p message to each other, which would have caused a deadlock previously (making this test fail), but succeeds now.

  As can be seen from the discussion in #27981, it was not easy to reproduce this bug without `sendmsgtopeer`. I would imagine that `sendmsgtopeer` could also be helpful in various other test constellations.

ACKs for top commit:
  ajtowns:
    ACK b3a93b409e7fb33af77bd34a269a3eae71d3ba83
  sipa:
    ACK b3a93b409e7fb33af77bd34a269a3eae71d3ba83
  achow101:
    ACK b3a93b409e7fb33af77bd34a269a3eae71d3ba83

Tree-SHA512: 6e22e72402f3c4dd70cddb9e96ea988444720f7a164031df159fbdd48056c8ac77ac53def045d9208a3ca07437c7c8e34f8b4ebc7066c0a84d81cd53f2f4fa5f

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