mrtest: add script to install packages from a MR

TODO:

• do not always cache the api call result to figure out which pipeline was the last one (doing that for development currently)
• show a warning that people should read the diff of the MR or at least trust the person who submitted it, ask for confirmation
• display which packages are already installed, add u letter for toggling those

After merging I plan to tag a version of this git repo and package it for alpine. mrtest will be in an extra package, so people can apk add mrtest it.

Other planned changes (maybe this MR, maybe in another):

• display artifact size when downloading it
• support alpine's aports