v3: Enum type support

[daniel-chambers]

Component

• GraphQL Engine
• ndc-postgres

Is your proposal related to a problem?

If I have a table in my database that represents a fixed enumeration of values, and I use that as a way to constrain the values of a column in a table to match a value from that fixed enumeration, I want my API to also expose this fixed enumeration to API clients so they know what valid values they can expect to receive/send.

Describe the solution you'd like

In Hasura v2, we supported reading the contents of a Postgres table and generating a GraphQL enum type from it. This type was then used as the type for any columns that foreign-key referenced that table in my GraphQL API.

I'd like a similar solution in Hasura DDN.