Add --list option to CLI to print the names of all available archivers

[krivard]

Overview

Closes #553.

What problem does this address?

What did you change in this PR?

• Add --list CLI option which prints each archiver name and then exits
• Update README to list a few sample archivers and then direct user to --list to see the rest.

Testing

How did you make sure this worked? How can a reviewer verify this?

$ pudl_archiver --list
[...]
$ pudl_archiver --help | grep list

To-do list

Tasks

Preview Give feedback

1. Update relevant documentation - like comments, docstrings, README, release notes, etc.
2. Review the PR yourself and call out any questions or issues you have

[aesharpe]

yay!