Add `grype db list-namespaces` for users to determine the available distros to use

With the addition of the `--distro` flag, it would be nice if a user could know exactly what values they should be using for this without having to somehow inspect the database, so we should add a command to list these, and consider updating the warning when no distro is found to tell them this command. Maybe something like:
```
grype db list-namespaces
```

Start at the existing `grype db` command and the `--distro` handling to trace how database namespaces are read. Add the `grype db list-namespaces` entry point so users can see valid distro values, and consider updating the no-distro warning to reference it. Done means the command lists the available namespaces and the warning guides users appropriately.

Written by the indexing model from the issue text.