Rust generator support for include list

[dbolduc]

We have an exclude list: skipped-ids. We would like an include list, maybe: included-ids.

We want to use the include list from the storage/v2 sidekick config that lives in storage-control. The set of RPCs/messages that should go into storage-control is fixed. It will not grow.

https://github.com/googleapis/google-cloud-rust/blob/499eec5a9985d68d1d8d3186b54fcf04189630cb/src/storage-control/src/generated/gapic/.sidekick.toml#L21

However, the set of media RPCs/messages can grow. Whenever a new message is added to that service, it will get surfaced in our API. We want to prevent these sorts of leaks.

https://github.com/googleapis/google-cloud-rust/blob/499eec5a9985d68d1d8d3186b54fcf04189630cb/src/storage-control/src/lib.rs#L43