sidekick: dedup Rust templates

We have a lot of duplicate templates, or at least templates that just load a partial from the right directory, e.g.:

https://github.com/googleapis/google-cloud-rust/blob/5bfc8d95e6e86df5e992546d64c0466c724ef054/generator/internal/rust/templates/crate/src/message.mustache#L16

After googleapis/google-cloud-rust#1888 we can use `{{> /templates/common/message.mustache}}` and tiny mustache files such as that one. But there are some exceptions, notably:

https://github.com/googleapis/google-cloud-rust/blob/5bfc8d95e6e86df5e992546d64c0466c724ef054/generator/internal/rust/templates/grpc-client/oneof.mustache#L16-L25

So one would need to avoid replacing `{{> oneof.mustache}}` blindly.

Inspect the Rust templates under generator/internal/rust/templates, starting with crate/src/message.mustache and grpc-client/oneof.mustache. Identify files that only load a partial through the common templates path, while preserving exceptions such as oneof.mustache. Done means duplicate wrapper templates are consolidated without replacing templates whose contents are distinct.

Written by the indexing model from the issue text.