[Draft Feature] Hook System for Binding Generation

[Togira123]

This draft aims to collect more feedback and ideas on a hook system for binding generation. The core idea is that users should be able to extend/modify the way the bindings are generated. The current implementation adds a new command to SCons, called binding_hook_file, which takes a file path to a python file. The specified python file should define a class that extends BindingGeneratorHooks and is called BindingGeneratorHooksExtension. It can then override the methods provided there to modify the way the bindings are generated. As an example, there is custom_generator.py included in this draft (won't be included in the final PR), which adds names of signals as constants for each header file. It can be tested using scons binding_hook_file=custom_generator.py.

The current implementation was already discussed here: https://chat.godotengine.org/channel/gdextension/thread/2gR6bngEu57tWLQG9