support libcamera v0.7.0 and vendored builds

[FreddyFunk]

This pr includes:

• Vendored build (meson submodule), so no need for locally installed libcamera
• Cross-compilation for aarch64, musl, riscv64 (meson cross-file generation, target-triple env vars)
• Feature gates (vendored, pkg-config, bindgen, pregenerated-bindings)
• Pre-generated bindings with generic opaque types for v0.7.0, arch-independent

I use this for libcamera support in my Camera app:
https://github.com/cosmic-utils/camera

closes #82
helps #17 and #52

[chemicstry]

Thank you for the PR solving the cross compilation pains

Could you take out the v0.7.0 additions? It is hard to review this PR with all the clutter, also #81 is now merged.

I like the vendored feature because it should also solve #16 I think? But I'm not sure if having entire libcamera as a submodule is a good idea, since vendored option should be an edge case. I haven't looked how other projects do this, but I have seen some that clone dependency into out directory and compile it there. Another problem with vendored build is that there are a lot of opinionated libcamera build configuration options that may not work for everyone and I don't think we can/should expose everything to be configurable through features/env vars. I will need some time to think about this

[Lishen1]

yes, better to clone libcamera on build step

[fishrockz]

I agree that #17 needs sorting but im not sure this does that.

This does not add any documentation.

I have been bad at working on this lib and helping with things like cross compilation. But thats partly because there are a lot of different ways todo it and lots of people do it just fine.

I use a SDK, lot of people use deb-bootstrap. given many people want to use this with a PI and the offical pi os then deb-bootstrap is a better way todo this than statically linking all of libcamera into a huge static bin.

I use a custom OS, so i use the custom OS's sdk so I'm not as familiar with deb-bootstrap and hence have not put the effort into documenting it.

But for most people deb-bootstrap is a better way to cross compile than creating a huge static bin like this will.