consider using a command line parsing library

[jsha]

Right now we depend solely on Go's built-in flag package. That's because so far our command line processing needs haven't been enough to justify a dependency for processing flags. However, a couple of things may change that consideration:

• rocsp-tool has a variety of subcommands (#6360) and we need a good way to make sure the documentation is up to date and the acceptable arg formats are consistent.
• boulder is now a single binary, dispatched by symlinking it with different names and executing it under various names depending on desired behavior. This requires an extra link.sh step in our build, and is a little obscure. It would be nice to migrate to a subcommand style for invoking the various boulder services.

[beautifulentropy]

One that I've had my eye on is github.com/urfave/cli (docs: https://cli.urfave.org/v2/getting-started). Compared to other popular CLI packages it has far fewer dependencies: https://github.com/urfave/cli/blob/main/go.mod