Add BrainAtlas hierarchy endpoint

[mgeplf]

Description

For this:
https://github.com/openbraininstitute/prod-explore-functionality/issues/267
Being able to get the volume so that regions can be greyed out is important.
At the moment, one would have to make a call to /brain-region-hierarchy/${id}/hierarchy
and then repeated calls to get all the metadata for the brain atlas.

Technical implementation & specs

Add "/brain-atlas/${atlas_id}/hierarchy"; it returns the hierarchy of that atlas, including the volume and is_leaf_region data.

Describe alternatives considered

Not having the endpoint; results in many calls.

Acceptance criteria

• "/brain-atlas/${atlas_id}/hierarchy"