Add support for head_repo when creating pull request (#3479)

I needed the ability to specify the head_repo during a create_pull call
in order to create a pull request on a different repo that is in the
same organization.

The change is relatively straightforward and just passes the value to
the post_parameters.

Resolves: https://github.com/PyGithub/PyGithub/issues/2942

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>