Create C# api generator script

[JCash]

Logical next step from https://github.com/defold/defold/issues/8924.

We now want to generate a C# api from our C api. As a test, we'll build an extension locally (i.e. not on the extender), to make sure that the api works.

Design doc: https://docs.google.com/document/d/1xITgxkLeBTOy5yP7rmqvrTcabe_iZrIJ_cW2DXTRcFo/edit?usp=sharing Subtask of https://github.com/defold/defold/issues/8063