Support for receiving message only if data is ready

[trepidacious]

Currently it seems to be hard to use one socket to both send and receive messages. I'm not sure whether it's actually possible to read messages that might have a gap longer than the connection timeout, since receive_message will block until it receives a message, which means you can't send a ping to keep the connection alive, leading to the issue in #36. #38 suggests that this can't be done with a select as recommended in #36, since it seems that data could be lost if the receive is cancelled after reading partial data.

I think I've got at least a partial solution to this - at least with some io implementations we will have ReadReady available as well as Read + Write, and this allows checking whether there is at least one byte of data available to be read immediately. This means we can implement a new receive_message_if_ready method - this will return immediately if there is no data available to read immediately, otherwise it will pass through to the existing receive_message method.

Technically this doesn't seem like a full fix - you could still have a case where a partial message comes through and then the rest is delayed, however I don't think this is really a problem. If you have a partial message then no more data received for more than a few seconds, it probably indicates that the connection or server is down and there's not much point in returning early from receive to e.g. send a ping, and I'd guess eventually the socket will either recover or close. It does fix the presumably much more common issue where everything is fine on the server, client and connection - there's just a long gap between incoming messages, but each message comes through relatively quickly after the first byte.

I'll submit a PR for the implementation, it doesn't really change anything much except adding the "if_ready" version of receive and poll methods, in the case where you have ReadReady available.

Side note - there is currently an issue with embassy-net where read_ready() is not implemented correctly for TcpSocket, this is fixed in main but not in the current 0.4.0 release. I've tested this approach on an esp32c3 with the fixed embassy-net and it seems to work fine for a loop with receive_message_if_ready() and send_ping(), staying connected overnight even if no messages are being received.