Peer Service crashes when message uses an unknown causal_label

[aramallo]

How to reproduce

(Partisan v5.0.0 master)

1. Start two partisan nodes using rebar3 shell
2. Get the PidRef for the Erlang shell on node 2 e.g.

1>partisan:self().
<<"partisan:pid:[email protected]:0.270.0">>

3. Send a message with a causal label that has not been defined in the Partisan configuration

1>partisan:forward_message(<<"partisan:pid:[email protected]:0.270.0">>, hello, #{causal_label => foo}).

Result

2023-01-26T19:51:19.447404+00:00 gen_server.erl:949 error: ** Generic server partisan_pluggable_peer_service_manager terminating
...
** Reason for termination ==
** {{noproc,
        {gen_server,call,
            [partisan_causality_backend_foo,
             {emit,'[email protected]',
                 {encoded_pid,"<0.270.0>"},
                 {'$partisan_padded',undefined,hello}},
             infinity]}},
    [{gen_server,call,3,[{file,"gen_server.erl"},{line,247}]},
     {partisan_pluggable_peer_service_manager,handle_cast,2,
         [{file,
              "/Volumes/Work/Lasp/partisan/src/partisan_pluggable_peer_service_manager.erl"},
          {line,1014}]},