Show player count as activity

[voruti]

This PR adds:

Showing the current amount of players connected to the proxy as Discord activity. The text/amount gets updated in method updateActivityPlayerAmount() which will be called when the bot is "ready" (initialize) or a player joined or left the game (regular updates). The activity text is configurable in the config.

Screenshot:

("Spielt" is "is playing" - Discord uses localized texts & my Discord client is setup with the German language.)

[voruti]

I thought about implementing a toggle in the config to be able to disable this feature - is this even necessary?

[fooooooooooooooo]

i like having things as configurable as possible, so a config option would be nice to have

[fooooooooooooooo]

thanks for the pr :)