Allow larger payload bodies than 10kB

[leio]

Some alternative servers, such as soketi, allow configuring the server to accept payloads larger than 10kB, however the code here will still reject it with

pusher-http-rust/src/client.rs

Lines 337 to 339 in 18ba21e

	if body.len() > 10240 {
	return Err("Data must be smaller than 10kb".to_string());
	}

and potentially other places.
Please consider removing that restriction, make it configurable or some other means to make use of larger payloads when the server supports it.