Added interface and extension methods to netcore

[rdvanbuuren]

Added additional templates which will generate the IFusionAuthClient interface and an extra FusionAuthExtensions class. This will keep the code better separated.

Changes:

• The interface itself only contains a description of the async functions and the documentation. This interface allows easier mocking and unit testing.
• The class only implements the async functions (sync methods are moved). Documentation is inherited from the interface.
• The extensions class continues support for the synchronous functions, which was previously added to the FusionAuthClient class.

The compiled code works just the same like before.

[matthew-altman]

@rdvanbuuren Thank you so much for the Pull Request. We really like the direction you're going here. We're accepting and merging this request. There will be some modifications in order to better support backwards compatibility and the client usage.