Add conceptType to featureContext constraint usage

[brendanreardon]

For both the adjacency and gene function constraint, can we specify the conceptType of mappable concepts that can be used? (either in the description or with an allOf in the schema)

Originally posted by @Mrinal-Thomas-Epic in #178 (comment)

Update usage of mappable concept within FeatureContextConstraint to specify a conceptType.

[brendanreardon]

I opened a discussion in the gks-core repo, here: ga4gh/gks-core#127