`rzup`: add option to list available versions

[matthiasgeihs]

Is there an existing issue?

• [X] I have searched the existing issues

Experiencing problems? Have you tried our Discord first?

• [X] This is not a support question.

Motivation

sometimes i just want to check what are the versions available through rzup. for example, is there a new version?

Request

Add a command --list (or short, -l) that lists all available versions.

Solution

No response

Are you willing to help with this request?

Maybe... (I have elaborated on how above)