Add brain region mesh router/schemas/service

[eleftherioszisis]

Allow querying brain region meshes using brain region ids.

[mgeplf]

I thought this was going to be part of the BrainAtlas work, so I've been putting somewhat equivalent functionality (along w/ the volumes) into the implementation for the spec: #168

[eleftherioszisis]

If this is going to be replaced by the BrainAtlas PR, I should close it. It was submitted before discussing of the new functionality.

[eleftherioszisis]

However, is it really equivalent functionality? Not sure how the sdks will be able to fetch BrainMesh as normal entities independently of a BrainAtlas. BrainAtlas provides a context for a subset of meshes per atlas, but why should it be disallowed to fetch meshes as regular entities?

[mgeplf]

IMO, a mesh is only useful within the context of a BrainAtlas (ie: you don't need to get all the meshes); as such, I think it should be modeled as an asset "attached" to a BrainAtlasRegion (ie: a region within the BrainAtlas)

[eleftherioszisis]

Having a BrainAtlasRegion with a mesh asset and other information created due to the atlas context makes perfect sense. I still think though we need to be able to also query them independently, but that can come later if needed.

I will close this PR.