build.rs support for cargo-component

hi folks,

playing with building wasm components and it seems the current approach requires one to run `cargo component bindings` to generate a `src/bindings.rs` file, which then has to be committed into the repository for whatever component (and re-committed on any changes).

a more common/rusty way to do this that avoids generated files being included in the source tree would be to [invoke the binding generation from `build.rs`](https://rust-lang.github.io/rust-bindgen/tutorial-3.html) then [import the generated bindings](https://rust-lang.github.io/rust-bindgen/tutorial-4.html) (like `bindgen` does).

have you considered supporting this approach? i see adding `build.rs` support was discussed in https://github.com/bytecodealliance/cargo-component/issues/362 but not actioned. it seems like generating the bindings from `build.rs` should be straightforward, then `cargo component` would need to pick up a flag or something to say that it shouldn't generate bindings itself.

cheers,

ryan