Add API generator JSON schema

[jnkvca]

Currently the rules for defining interfaces (classes) for API generator tool are all residing within the README.md document.

It would be great if we could turn that into an actually JSON schema. AI could be of help here, but then we need to fully manually cross check the output before calling it done.

Furthermore, we can actually integrate this schema into our VS Code workspace (some config within .vscode dir probably) and actually make use of its autocomplete and intellisense.