[GDExtension/CMake] Add shared Godot/Godot-cpp CMakeFile for use via FetchContent

[LouChiSoft]

I know CMake support is new and isn't as mature as SCons, but as it get better I think more people are going to prefer it over SCons. Right now from my experience if I want to make a GDExtension with CMake I need to manually get, build and run a matching version of Godot to dump the extension_api.json to then pass it into the godot-cpp CMakeLists for making the GDExtension.

My proposal is a sort of CMake based "glue file". The idea would be that you can use via methods like FetchContent and gets and builds both the Godot engine and the godo-cpp library. It would sync the settings between the projects, precision, engine version, etc. and provide a target for the godot engine being built for a convenient way to test the GDExtension being developed.

In theory if done well, the user could include the CMakeLists.txt then using a custom command add targets for each combination of engine settings. e.g. one target for single precision and one target for double precision, as well as generating a gdextension file based on the outputs of the project. Also a user updating a GDExtension would be as simple as updating the branch that is used to build Godot and godot-cpp.

Pseudo-code example usage:

# Standard project setup
project(my-gdextension LANGUAGES CXX)
    set(CMAKE_CXX_STANDARD 17)
    set(CMAKE_CXX_STANDARD_REQUIRED ON)

    # Include new CMake project files
    include(FetchContent)
    FetchContent_Declare(
        godot-cmake
        GIT_REPOSITORY https://github.com/godotengine/godot-cmake.git
    )
    FetchContent_MakeAvailable(godot-cmake)

    # Define GDExtension target
    file(GLOB SRC "src/*.cpp")
    add_library(${PROJECT_NAME} SHARED ${SRC})
    target_include_directories(${PROJECT_NAME} PRIVATE "${CMAKE_SOURCE_DIR}/src")

    # Link to Godot and Godot-CPP
    link_to_godot(
        TARGET ${PROJECT_NAME} 
        PRECISION "single"
        # other features
    )
    
    # Same again, but this time for double precision
    link_to_godot(
        TARGET ${PROJECT_NAME} 
        PRECISION "double"
        # other features
    )

    # Generate a GDExtension file
    generate_gdextension_file(
        TARGET ${PROJECT_NAME}
    )