Allow schema to express edge cardinality constraints

[obi1kenobi]

When generating a GraphQL schema, we should express edge cardinality constraints: if an edge is guaranteed to be unique, the type on the other side would be Foo, and if it is not unique, it could be [Foo] etc.

Some ideas as to how this could be done:
http://blog.liu.se/olafhartig/documents/graphql-schemas-for-property-graphs/
http://olafhartig.de/files/HartigHidders_GRADES2019_Preprint.pdf
https://github.com/LiUGraphQL/pgschema-to-apischema/wiki/Overview-of-the-Approach

Originally discussed in #284, and somewhat related to #274.