Adds OnPushRemoteProgress handler to PushOptions

[sitereactor]

The NativeMethods for receiving text updates from the remote already exists and are used in the FetchOptions. This simply adds the existing handler to PushOptions making it possible to configure a callback for receiving text updates from the remote.

No tests have been added for this PR, as its not possible to get remote updates when pushing to local bare repositories. I have integration tests locally that shows this working, but a remote repository is needed for it to be successful. Hoping that the simplicity of this makes it straight forward to merge.

This is discussed here https://github.com/libgit2/libgit2sharp/issues/642