Add a separate setup-godot-cpp github action.

[Ivorforce]

As discussed in the godot-cpp meeting. This action is specifically intended to be used by dependants of godot-cpp, to set up godot-cpp in their own github workflows. A follow-up PR will use this github action in godot-cpp-template (edit: godotengine/godot-cpp-template#70).

There are quite a few differences to godot-cpp-template's setup steps. I don't know if they started the same but diverged later, or if godot-cpp-template has always made different decisions from godot-cpp itself. Here is godot-cpp-template's version for reference.

I based my implementation mostly on godot-cpp. Here are the key differences I see:

• actions/setup-java@v4 and android-actions/setup-android@v3 are used on godot-cpp-template, while nttld/setup-ndk@v1 is used on godot-cpp.
  • godot-cpp-template also manually edits a bunch of variable files to get android set up.
• emsdk caches are used by godot-cpp-template, while they are explicitly disabled on godot-cpp. This was recently decided in [Web] Don't cache emsdk #1639 and should not be reverted.
• godot-cpp-template installs build-essential pkg-config on linux. godot-cpp only installs these dependencies for the cmake build. I conclude they may not be needed for scons builds.

I think that's all of the differences, though the github runner shall smite my PR if it's not compatible anymore.

[Ivorforce]

Looks like caches are still working, plus the tests in godotengine/godot-cpp-template#70 have succeeded (especially, the Android ones which I was unsure of). I'm taking this as a good sign that this PR retains cross compatibility!

[dsnopek]

Overall, this seems good to me! It's really just moving a small amount of stuff from the workflow into an action

[dsnopek]

Thanks!