Add API version configuration

[lukellmann]

This PR allows to configure which versions of Discord's APIs Kord uses.
This is achieved by changing the properties REST_VERSION, GATEWAY_VERSION or VOICE_GATEWAY_VERSION of the KordConfiguration object:

object KordConfiguration {
    var REST_VERSION: Int
    var GATEWAY_VERSION: Int
    var VOICE_GATEWAY_VERSION: Int
}

Reading the REST version from the environment variable dev.kord.rest.version is removed.

DefaultGateway.Companion.invoke now no longer uses an unversioned gateway url.

[lukellmann]

This is a first draft of the configuration options I mentioned here.

Would be nice to get some feedback on this. Are there more things that could be configured in this way?
(@HopeBaron @DRSchlaubi @MrPowerGamerBR)

[lukellmann]

If there are no concerns about the design of this (mutable properties in an object), I would say this is ready for review.

[HopeBaron]

Looks fine 👍

[lukellmann]

This PR removed the separately listed Gateway version, should we also only have one property for this?

[lukellmann]

This PR removed the separately listed Gateway version, should we also only have one property for this?

I thought about this and we probably shouldn't - if you want to change the version for only one of them you should be free to do so.

[HopeBaron]

Do you mean to have a fixed set of versions?

[lukellmann]

I meant always using the same version for gateway and rest, but I think we should allow changing them independently.

[HopeBaron]

Yeah it looks fine; we just need to fix the conflicts to go

[lukellmann]

Will do today/tomorrow now 👍

[lukellmann]

Done