Prebuilt Godot artifacts

[Bromeon]

Closes #12
Closes #107

From now on, gdext by default fetches pre-generated versions of these files, published in the godot4-prebuilt repo:

• extension_api.json (from Godot binary)
• gdextension_interface.h (from Godot binary)
• gdextension_interface.rs (through bindgen -- currently supports 3 platforms)

This has several benefits:

1. Significantly fewer dependencies, as bindgen is no longer needed, and thus smaller compile times.
2. Most CI jobs no longer need the Godot binary (clippy, test), speeding up CI in addition to (1).
3. It's possible to change the Godot API behind gdext without manually rebuilding the artifacts.
4. Easy comparison between the Godot APIs of different released versions.

Using a custom Godot binary

It is still possible to generate those files locally like before, through the use of the custom-godot feature on the godot crate.

This is necessary for any platform/configuration different from the 3 main supported ones (because bindgen generates different Rust bindings), as well as any in-development or modified Godot versions.

Changing the Godot release

By default, the latest Godot release is used as input to gdext. Switching between different Godot versions is easily possible, although a bit cumbersome.

If you want to use an older version 4.0, add this to your workspace (not sub-crate) Cargo.toml:

# We need to trick Cargo into seeing a different URL; https://github.com/rust-lang/cargo/issues/5478
[patch."https://github.com/godot-rust/godot4-prebuilt"]
godot4-prebuilt = { git = "https://github.com//godot-rust/godot4-prebuilt", branch = "4.0"}

We're looking into ways to simplify this. In a crates.io release (#2), we would need to rethink this anyway, mapping Godot versions to Rust release versions (which is not trivial).

[Bromeon]

bors try

[bors]

try

Build failed:

• full-ci

[Bromeon]

bors try

[bors]

try

Build succeeded:

• full-ci

[Bromeon]

bors r+

[bors]

Build succeeded:

• full-ci