fix webrtc example that relied on JSON integers

[goatchurchprime]

This fixes the problem identified by godotengine/godot#103374

The JSON.parse_string() creates floats instead of integers from numbers, which the example flagged as an error. The solution is to send integers as strings instead.

[aaronfranke]

Isn't this a workaround, not a solution? JSON numbers are, in fact, floats. Any check like str(msg.type).is_valid_int() in the ws_webrtc.client._parse_msg() function is just not a good check, right?