Split extension methods into separate classes

[Octol1ttle]

This PR splits the extension methods contained in Extensions.cs into separate classes in the Octobot.Extensions namespace. This was done for multiple reasons:

1. The Extensions.cs violates SRP (Single Responsibility Principle) - it takes upon itself every extension method for many types
2. Having a separate class for each extended type is a standard practice - take a look at Remora.Discord or osu!
3. Having all extension methods in one file makes it hard to find the method you want