Discovering Postgres enum

[tyt2y3]

So to support SeaQL/sea-orm#252

We need to properly discover Postgres enums from a given schema (MySQL enum is done already)

For example, given a schema:

sea-schema/tests/sakila/postgres/sakila-schema.sql

Lines 64 to 70 in e79afd4

	CREATE TYPE mpaa_rating AS ENUM (
	'G',
	'PG',
	'PG-13',
	'R',
	'NC-17'
	);

We want to query the enum definition from information_schema and then output a TypeCreateStatement in the end:

Type::create()
    .as_enum(Alias::new("mpaa_rating"))
    .values(vec![
        "G",
        "PG",
        "PG-13",
        "R",
        "NC-17",
    ])

https://github.com/SeaQL/sea-query/blob/HEAD/src/extension/postgres/types.rs

We may need to add new data structures to the def module too.

Once we got the writer done, we can transform that in SeaORM inside the codegen.

[tyt2y3]

@charleschege