Introduce some interactivity to artifact cache expiry

Background

Currently, when a build pipeline does not fit inside the artifact cache quota, buildstream will unconditionally give up. This is alright, but perhaps annoying to the user.

Task description

In interactive mode, buildstream should prompt the user, allowing them to temporarily increase the disk size if they want to - this should have three options:

• Abort the build, keeping all artifacts (default)
• Abort the build, purging some artifacts used in this build
• Continue building, ignoring artifact cache quota for this build

Acceptance Criteria

We have a nicer UI for running out of cache space.

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