Add some official cli command for compute_auth_tag

To start agents in a container on another machine not reachable by the desktop app, I have to create an auth tag like this:

```
cargo run --release --example compute_auth_tag --
```

This involves having the source checked out and built. Whereas to generate the agents keys, I can just invoke the CLI in a throwaway container like this:

```
docker run --rm --entrypoint /usr/local/bin/buzz-admin ghcr.io/block/buzz:main generate-key
```

It'd be nice if creating an auth_tag was also available in a precompiled CLI binary somewhere, or in the `run.sh` script in the `deploy/compose` script, to avoid having to build ourselves.