Add max_message_size as a configurable connection parameter

[sebastian-king]

The WebSocket library used has a default max message size of 4MB (per https://docs.aiohttp.org/en/v3.8.4/client_reference.html#aiohttp.ClientSession.ws_connect).

If the NATS server supports and has messages larger than this 4MB limit, we encounter the following error during reading of the message:

nats: encountered error
Traceback (most recent call last):
  File "/home/seb/.local/lib/python3.8/site-packages/nats/aio/client.py", line 2073, in _read_loop
    await self._ps.parse(b)
  File "/home/seb/.local/lib/python3.8/site-packages/nats/protocol/parser.py", line 95, in parse
    self.buf.extend(data)
TypeError: can't extend bytearray with NoneType

This error basically translates to, self.buf is smaller than the size of the payload b (within the AIO WebSocketReader class).

As such, I've added a new max_message_size property to the connection, which is passed to the WebSocket transport to resolve this issue. I maintained the default 4MB limit.