Support P2P Connections with Custom Signaling Between Steam and OSS Library Versions

[lukaasm]

As a user, I would like to establish P2P connections using custom signaling between the Steam version of the library and the OSS (Open Source Software) version.

However, the Steam version currently disallows unauthenticated/unsigned P2P communication, which prevents this interoperability.

EUnsignedCert CSteamNetworkConnectionBase::AllowRemoteUnsignedCert()
{
	#ifdef STEAMNETWORKINGSOCKETS_OPENSOURCE
		// We don't have a cert authority.  We probably ought to make this customizable
		return k_EUnsignedCert_Allow;
	#else
		return k_EUnsignedCert_Disallow;
	#endif
}

[11:35:39.700] [warning] NetInterfaceGNS.cpp:188 - Failed to start accepting P2P connect request from str:393FD8571292475BB1128DCC7D73745 on vport 0; Unsigned certs are not allowed

Ideally it could be controlled with new config var for non-SDR connections.

k_ESteamNetworkingConfig_P2P_AllowWithoutAuth