Add proxy options

[bording]

This PR adds a ProxyOptions class that can be used to specify proxy options to all of the relevant places where a proxy could be needed.

ProxyType is set to Auto by default, so LibGit2Sharp will now respect any proxy settings configured in git. ProxyType can be set to None to disable using the repo's proxy settings. Specified let's you specify a a proxy URL. There are also callbacks to deal with credentials and certificate verification.

As part of these changes, CloneOptions and SubmoduleUpdateOptions have been updated to have all fetch-related options be exposed via a FetchOptions property and not a confusing mix of some being directly on the class and others being only in an existing FetchOptions property.

I haven't added any proxy-specific automated tests here primarily because I don't currently know of a good way to have a proxy server spun up in GitHub Actions that I can reliably use on all platforms that need to be tested on.

For now, the manual testing I've done will have to be enough!

[JonWillis-Tote]

Hi, This was my previous code, but this PR as mentioned is breaking.
Is there documentation on how to use the new proxy feature so it works

 var cloneOptions = new CloneOptions();
 var usernamePasswordCredentials = new UsernamePasswordCredentials { Username = _githubOptions.Username, Password = _githubOptions.Token };
 cloneOptions.CredentialsProvider = (_url, _user, _cred) => usernamePasswordCredentials;
 Repository.Clone("https://github.com/Jon/MyRepo.git", directory, cloneOptions);

As CredentialsProvider has been removed, I did try replacing it with this, but it failed.

cloneOptions.FetchOptions.ProxyOptions.CredentialsProvider = (_url, _user, _cred) => usernamePasswordCredentials;

LibGit2Sharp.LibGit2SharpException: 'remote authentication required but no callback set'

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Edit, Solved it. Given this change was about a proxy, i used ProxyOptions, but it turns out I didn't need it after all.

cloneOptions.FetchOptions.CredentialsProvider = (_url, _user, _cred) => usernamePasswordCredentials;