Support Postgres views

[ramnivas]

Developers should be able to specify types bound to a Postgres view. From the GraphQL API point of view, a view will work effectively as a read-only table.

Initially, we can support views whose definition is externally controlled (i.e., the exo schema create etc. commands will ignore such types). Later, we can extend this to allow users to specify the definition of the fields in a view type.