Simplify metadata within Artifact class

Background

Now that !1271 (merged) has landed, we have somewhat simplified that internal handling of artifact metadata by internalising it into the Artifact class from the element, removing the need for accessor methods and less than ideal parameter handling. This simplification should be taken further now that the Artifact instance within element represents a single artifact, with the keys internalised removing the need to have keyed dicts for the metadata.

It's also worth noting that the storing of metadata within the aritfact changes once we switch to Artifact as a Proto, removing the need for expensive yaml extraction and parsing.

Task description

• Rework the get_metadata* methods and member variables for the above simplification

Acceptance Criteria

No breaking changes, simpler handling of data

---