fix(types): fix enum tables when used in custom mutations

[benjie]

Description

@brendanmckenzie reported graphile/crystal#1365 pointing out there's an issue when types that reference enum types are used in custom mutations. This is because we explicitly cast the type of each column, and when we were casting the enum to it's fake name it happened to overlap with the table name, so PostgreSQL thought it was a table reference. Really it should have been a cast to ::text or whatever type the enum column is.

For now I've solved this by casting to ::unknown if the type is fake; but we should solve it better in the long run by having the PgType have a sqlFullyQualifiedTypeName field that we can override for fake types to be the type of the column on the enum (e.g. pg_catalog.text).

Fixes graphile/crystal#1365

Performance impact

Negligible.

Security impact

Fixes a bug.