Make owner validation configurable

[ltrzesniewski]

This adds a GlobalSettings.OwnerValidation property which wraps git_libgit2_opts(GIT_OPT_[GET/SET]_OWNER_VALIDATION).

I needed to disable this (I'm using a safe environment), and I had to do this through reflection, but I suppose other people may have the same problem so exposing the feature in the library would be better. 🙂

Closes #2036

Edit: I'm an idiot, I forgot to check whether someone else submitted a PR for this, so this duplicates #2042, sorry 🤦
I'm leaving this open so you can choose the implementation, and this one also lets you retrieve the current option value.

[bording]

@ltrzesniewski Thanks for the PR. Of the open PRs for this feature, yours was was the most complete (it even included a test!) so I decided to it to get this feature in.

I made a few tweaks, primarily moving away from a property and instead having separate get/set methods. Overall I think separate methods make more sense when there are separate libgit2 methods that need to be called.