Implement support for injecting events

[knah]

I have no idea why this would be useful, but some reflection-based libraries might make use of this. Events might be a bit tricky due to generated delegate types being not-really-delegates, so a managed-side event can't have an IL2CPP delegate type and vice versa. Due to that, this might be non-implementable in general, unless some additional hoops are jumped, similar to those needed for field injection.