Move push/pull/checkout to the artifact subcommand group

Description

This MR aims to move bst push -> bst artifact push, bst pull -> bst artifact pull and bst checkout -> bst artifact checkout.

Note that:

• The artifact subcommand group currently provides bst artifact log.
• The current behaviour of push and pull will not change with this MR, they will still only be compatible with element names (not artifact refs).
  • In order to make them work well with artifact refs, some rework of how we handle artifact refs (instead of elements and keys) throughout the code base must first be completed.
• The behaviour of checkout will change w.r.t. the fact that LOCATION is no longer a positional argument. Instead, a mandatory --directory (or --tar) option must be specified. In addition to this, checkout can only handle one element (like it can currently), this MR is a step towards handling multiple artifact refs.

Partially addresses #822 (closed).

Proposed changes

Changes proposed in this merge request:

1. Move pull and push so that they're part of the artifact subcommand group.
2. Move checkout to the artifact subcommand group
3. Ensure the tests are passing.
4. [ ] Ensure that pull and push can take artifact refs.

Side note

4 is no longer required within the scope of this MR, work is currently being undertaken to address this, but it is not a blocker for this MR.