Use mbedTLS instead of OpenSSL

[Faless]

Build dependencies using cmake.

I was nerd-sniped by the merge of #191 and #199 to try and switch to mbedTLS as the crypto library.

This should make it much easier to get ARM32/ARM64 (#260) builds on both windows and linux (I've confirmed cross-builds for linux arm32/arm64, for windows, see godotengine/webrtc-native#174 ).

I can add the CI bits as a separate commit to this PR if desired.

[akien-mga]

The link is no longer accurate. This one is better:
https://github.com/godotengine/godot/blob/master/platform/android/java/app/config.gradle#L16

And we're now on NDK r28 upstream, though I don't know how in sync the Git plugin needs to be...

Actually, we don't build this plugin for Android, so I guess this is just copied from another cross-platform repo?

[Faless]

Yeah, this comes from the webrtc-native repository.

I've been wondering how we can maintain GDExtension compatibility.... what happens if we build with a different NDK version? Is targeting the lowest NDK guaranteed to work? AFAIK using a newer version results in the library failing to load.

[akien-mga]

Seems good to me. If anything we're more familiar with mbedTLS and it works well for Godot, so it's a better pick IMO.

As discussed with @Calinou, we'll first make a release with the new OpenSSL PRs merged (and Godot 4.2 baseline), and then merge this PR and make another release.

So there will be a recent release with OpenSSL and Godot 4.2 to compare with in case there are any new issues related to mbedTLS.

[Calinou]

@Faless Can you look into rebasing this PR? The SSL certificate issue is still being run into regularly with 3.2.1, so I'd like to merge this, tag 3.3.0 and hopefully fix the issue once and for all.