Automate common library maintenance tasks

[coryan]

We will need to repeatedly run the generator to keep the protos up to date. The generator should gain some subcommands so we can run things like:

• [x] Generate a library and save the command-line flags used to generate it (I am thinking a .sidekick.toml file).
• [x] Refresh / regenerate a client library using the saved command-line arguments.
• [ ] Support a repo-wide set of settings for all directories
  • For example, the language is rust in the google-cloud-rust directory.
  • Accept per-directory overrides for tests and other edge cases.
• [ ] Support https://github.com/googleapis/googleapis@{SHA} (or something similar) as a source for protobuf.
  • sidekick should download the specified version in that case, and provide the path to each directory.
  • the generated directory should use the global setting for this.
• [ ] Refresh / regenerate all the client libraries using the saved command-line arguments.
• [ ] Update the googleapis SHA and regenerate all the client libraries.

[coryan]

The generate and refresh commands work. generate saves the options in a .sidekick.toml file, that refresh can use to do its job.