Add `mqttrs::decode::check()` method, to see if the packet is decodable?

[00imvj00]

The idea here is, before trying decoding and then returning Ok(None), what if we can create diff method, check() -> bool which will tell if the full packet is received or not based on the remaining_lenth?

[MathiasKoch]

I don't see why that wouldn't work.

[00imvj00]

This will work. This is just a feature requirement.