Specify allowed values for entities that requires specific values in bids schema

[soichih]

Ref >

bids-specification/src/schema/entities.yaml

Line 169 in c043412

Allowed label values for this entity are `phase`, `mag`, `real` and `imag`,

Would it make sense to enumerate all allowed values for certain entities like this?

    options:
      - phase
      - mag
      - real
      - imag 

Downstream apps (ezBIDS, bids-validator, etc) can then use this information to let user pick / validate from one of the allowed values?

[tsalo]

Ah, I actually have a PR for this: #792. It's a bit more expansive than this issue, but could be scaled back if necessary.