Add api_version build option to select the API JSON file

[dsnopek]

This builds on top of #1895, and is a replacement for our usual update of the extension_api.json file for Godot 4.6 (which we usually start doing at -beta1, so this also adds the extension_api.json file for 4.6-beta1)

The idea, is that we'll include extension_api.json files for all the Godot versions that godot-cpp v10 is compatible with, and developers can specify scons api_version=4.5 (for example) to select an earlier API version. This is something that other GDExtension bindings do as well, for example, the Rust bindings.

This is marked as a DRAFT because it depends on #1895 (the first godot-cpp v10 PR) which allows godot-cpp to be compatible with either Godot 4.5 or 4.6

[dsnopek]

After working on #1898 and #1899, I'm seeing how tricky it is to test these, so I think we should have the CI test all supported versions, and this PR is a good one to start that in

So, that's something I'll add when I have a chance!

[dsnopek]

I've updated the CI so that it'll build for each supported API version (only 4.5 and 4.6 in this PR - more are added in the follow-up PRs), and then run the tests using each supported version of Godot.

So, if it builds for 4.6, it just tests that against master. But if it builds for 4.5, it'll run the tests on 4.5 and then master. And in the PR that adds support for 4.4, it'll run that build against 4.4, 4.5 and master. And so on...

This is all I plan to add to this PR, however, it'll remain marked as DRAFT because the PR it depends on still isn't merged.

[dsnopek]

Turns out I had one more thing to add: cmake support!

Seems to work in my testing, but I really don't know what I'm doing with cmake :-)

cc @enetheru

[enetheru]

Seems to work in my testing, but I really don't know what I'm doing with cmake :-)

Looks good to me. None of the changes to CMake add additional complexity, nothing raises my eyebrows. :)

[dsnopek]

Rebased and taken out of DRAFT now that #1895 is merged

[Ivorforce]

LGTM

[Ivorforce]

Is that expected? Otherwise a TODO might be good

[dsnopek]

It was expected - at one time Godot's first time import of the test project was crashing. I'm not sure if that's the case anymore (or which version it crashed with), but since we're going to be running this with older versions of Godot (not just the latest) we may need it. I think we should keep it for now, since we're not even entirely sure how far back we're going with Godot versions