Moved rust extension rules into a separate rules_rust_ext workspace.

[UebelAndre]

This change moves the bindgen, proto, and wasm_bindgen sub-packages into extensions which is isolated into it's own workspace called rules_rust_ext. The intent is improve ease of maintenance of both core and extension Rust rules by ensuring changes to extensions have no impact on the core rules. Core rules should never depend on extensions.

Load statements should be updated according to the following table:

before	after
@rules_rust//bindgen	@rules_rust_ext//bindgen
@rules_rust//proto/prost	@rules_rust_ext//prost
@rules_rust//proto/protobuf	@rules_rust_ext//protobuf
@rules_rust//wasm_bindgen	@rules_rust_ext//wasm_bindgen

closes #2882

[UebelAndre]

@illicitonion @krasimirgg @hlopko I would love your input on this change. It's a massive breaking change but I think a better path forward to help ensure the protobuf/rules_cc headache from #2998 doesn't impact the core rules again.

[krasimirgg]

Thank you I think it looks very reasonable, but also I'm not working in an environment where rules_rust+bzlmod is exercised, so it's hard to flag potential non-trivial issues with this. I just asked a few folks to take a look at this and share their thoughts.

[konkers]

Curious about the motivation to put them all into a single @rules_rust_ext module. I would have naively expected them to have separate modules (i.e. @rules_rust_bindgen, @rules_rust_prost, etc.) That way a project would have more fined grained control over it's dependencies.

[UebelAndre]

Curious about the motivation to put them all into a single @rules_rust_ext module. I would have naively expected them to have separate modules (i.e. @rules_rust_bindgen, @rules_rust_prost, etc.) That way a project would have more fined grained control over it's dependencies.

I started down that path and hit the 80 job cap for BazelCI. For each extension I would want to test them on Linux, Linux RBE, MacOS, and Windows, and to have each product be it's own workspace didn't seem possible. Though @meteorcloudy maybe there's a way to raise that cap?

edit:
But the trade I took here was to at least be able to isolate ruless_rust from bulky external dependencies. If there's a path to granular modules that doesn't involve super slow or no CI, I'm happy to take it!

[illicitonion]

Happy to do this, equally happy to have a per-extension module if that's practical :) Thanks for doing the work!