[Postgres] Order ENUM values

[jmonticolo]

Description

When a vector layer with Postgres / PostGIS provider with an ENUM type field is edited, the values are loaded with the following query :

SELECT enumlabel
FROM pg_catalog.pg_enum
WHERE
    enumtypid = (
        SELECT atttypid::regclass
        FROM pg_attribute
        WHERE
            attrelid = '"table_schema"."table_name"'::regclass
            AND attname='field_name'
    )
;

In the great majority of cases, the ENUM values are read in the right order, but not always.
This PR adds ORDER BY enumsortorder at the end of the query.

Impact : the request is slightly longer (the quicksort adds few milliseconds).

[github-actions]

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 8260527)

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 8260527)