Introduce `bst artifact show`

Description

This MR introduces the new subcommand bst artifact show. This command shows the cached status of an artifact and supports both artifact refs and element names.

If project/user remotes are available, we check the whether each target (and it's deps, if specified) are available in any of the remotes. Uncached, but remotely available artifacts are displayed as "available".

For more information, see #1103 (closed)

Important note

When I proposed the design on the ML, I initially suggested that I'd list all available artifacts for an element. I have decided not to implement this for show, instead, I think this would better belong in a new artifact list subcommand.

Possible extensions

• Implement bst artifact list
• Implement some kind of --format option