Consider writing storage-control surface via custom `build.rs`

[dbolduc]

Copying RPCs into the storage-control client surface is rote. We do it once when an RPC is added.

This is a manual process. It would preferably be automated as part of updating googleapis.

Also, there is nothing updating the client documentation. It is only copied once.

We could fix these problems with a custom build.rs that writes the client.rs surface for us, as well as the stub.rs surface.

Note that we expect the need for this feature to be temporary. That is why we think customizing build.rs is appropriate, vs. adding specific support in sidekick for this.