Support other Motd/Advertisements

[Adrian8115]

The currently supported motd is the motd used by mojang, but other games might use another motd structure.

This could easily be added with a simple enum like:

enum Advertisement {
   MinecraftBedrock(McMotd),
   Other(String),
}

This would also allow other advertisements to be supported, from other games/applications.

[john-bv]

I like this idea, however I will more than likely isolate MCPE behind a feature flag like it is currently. I'm also not sure if I would do this with a enum, and i'd probably go more with a trait. I'll look into this.