Add getters for protocol version and name; default to "MQTT" for name

[jeff-hiner]

MQTT broker implementations are required to verify that the protocol name and version are correct before accepting connections, so this PR provides a way to access those fields.

I've also removed a field from ConnectPacket::new that doesn't really make sense: the protocol name for 3.1.1 going forward must always be "MQTT". Users can still use with_level if they want to specify version and name manually (i.e. to use "MQISDP" for a 3.1.0 client, or to provide invalid sequences for broker testing). This is a breaking API change, but updating should be pretty trivial for most users.

[zonyitoo]

We shouldn't make the protocol level fixed as an enum, it would be nice to define some helper functions like:

impl ProtocolLevel {
    pub fn protocol_3() -> ProtocolLevel {
        ProtocolLevel(0x03)
    }
}

already good enough.

[jeff-hiner]

From trying to write a broker I find it's more ergonomic to not even expose those inner const values-- I don't care what the inner u8 is for those and trying to remember which is which gets confusing. I just need to know what protocol version is specified (the number of legal values is very small, and they unfortunately aren't all in a single spec document). It's unfortunately not trivial because the u8 doesn't even map neatly to the MQTT version-- spec 3.1.1 is 0x03, and for some reason they skipped spec 4 and said spec 5.0.0 is ProtocolVersion 0x04 (?) So without this parsing into an enum I'd have to do some sort of lookup by const values. It's just interpretation that I feel works best within this crate rather than the broker.

I could add #[non_exhaustive] to indicate that there may exist more variants than we expose, but in practice virtually every broker and client right now are either 3.1.1 or 5.0.

[zonyitoo]

and for some reason they skipped spec 4 and said spec 5.0.0 is ProtocolVersion 0x04 (?)

https://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html#_Toc385349227

Nope. v5.0 is 0x05.

[jeff-hiner]

With a bit of a chuckle at my own mistake, I think you just made my point. I don't want to think about this stuff, and the field is essentially a de-facto enumeration of repr u8. Can we just parse it into an enum? It'll make my MQTT broker project a lot easier to write.