refactor: data-driven backend project templates

[ericswanson-dfinity]

Description

Refactors the code that hands backend project templates to be data-driven rather than hardcoded as an enum.

Part of https://dfinity.atlassian.net/browse/SDK-1805

How Has This Been Tested?

Most is covered by existing e2e.

I added two tests that show that the help and command error output still include the list of possible values. It might seem like these are just making sure that clap works, but an experiment in which I tried to make the type variable an Option<ProjectTemplateName> turned out to not show the possible values.

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [ ] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.