Add ratelimiter to allow custom hard-coded global ratelimits

[Bastian]

On larger bots, it is possible to hit the global ratelimit of 50/1. Preventing to hit this global ratelimit is not possible with just the ratelimit HTTP-headers that we receive from Discord, but only with hard-coded limits. As the limits could change over-time (and there might even some increased limits for large bots?) it's no option to hard-code them directly into the Javacord code.

This PR allows users to set their own ratelimits in the DiscordApiBuilder. A default implementation is available for local ratelimits that allows users to set a hard-coded limit. It is possible to write custom implementations that allow more complex ratelimiters that can for example share their state across multiple machines.

The code can be used like this:

DiscordApi api = new DiscordApiBuilder()
        .setGlobalRatelimiter(new LocalRatelimiter(50, 1)) // 50/1 ratelimit
        .setToken("<token>")
        .login()
        .join();