Implements godotengine/godot#86079 in Rust, enabling required parameters and return types.

GDExtension newly advertises information about parameters and return types in engine APIs, which are required, meaning non-null. In Rust, this means we can use Gd<T> instead of Option<Gd<T>>, improving type safety and unnecessary unwrapping.

The generated code looks as follows:

The upstream PR is not yet merged, but you can test it as follows:

• Check out latest commit of that PR (currently godotengine/godot@9ccb329).
• Build Godot in dev/editor mode locally.
• Set GODOT4_BIN environment variable to the Godot binary.
• Check out this branch of godot-rust and compile, see below.

I gated this functionality behind a new feature experimental-required-objs, because it's still in flux, and every upstream change becomes breaking. Since I also refactored a few parts in related codegen, I will merge this soon, but the feature should leave existing usage unaffected.

Compilation:

# Integration tests
cargo build -p itest --features codegen-full-experimental,godot/api-custom,godot/experimental-required-objs
$GODOT4_BIN --headless --path itest/godot

# Unit tests
cargo test --features godot/api-custom,godot/experimental-required-objs

# RustDoc generation
cargo doc -p godot --features api-custom,experimental-required-objs,__codegen-full --no-deps --open