feat: Reorganize codegen and generated API files

[mkundu1]

1. The codegen module is moved to src/ansys/fluent/core so that it is packaged. A helper script is added at codegen/allapigen.py for users.
2. All API files will be generated under a single directory, src/ansys/fluent/core/generated for PyFluent. This is to make packaging of the API files in Fluent side easier. I think this is okay as users won't directly import these files. The generated API directory is controlled through a module level variable which will be overwritten in Fluent side.

This PR contains the high level changes of the source and packaging structure. The goal is to package the codegen module and use it in Fluent to generate the API files at some Fluent packaging path. There will be additional PRs to refactor the codegen code to use Fluent's local static info. We shall also create a zipped module for settings API files to include in Fluent package (not for PyFluent ATM as the expanded directory is useful as the .pyi files are read by intellisense).