Register custom filters

I want to expose an API for registering custom clean and smudge filters via libgit2.

My knowledge of C is rusty, and this is the first time I have looked at libgit2sharp in any great detail. So, I'm going to open this PR early, in the hopes that if anyone has time to spot anything terrible I do along the way they will be able to point me in the right direction :pray: 

###  Todo

- [x] Register and deregsiter a filter
- [x] Define apply callback
- [x] Define check callback
- [x] Define shutdown/init callbacks
- [ ] Tests
thanks for the review @nulltoken :heart_eyes: No I don't think we are going to need this - it's still hanging around from the work in progress around providing a friendly way to modify the blob stream as it passes through the filter. I'll make sure to remove it :+1: