me: fix regression w/ mapped enum names on postgres

When calculating a SQL schema from a Prisma schema, the Postgres
connector of the migration engine would use the name of the enum instead
of its database name (see parser-database docs for the reference on what
these mean). To picture this with an example, the following schema.

```
enum PersonType {
  Dog
  Cat
  @@Map("pet_preference")
}
```

would correspond, before the regression, and after this commit, to:

```
CREATE TYPE "pet_preference" AS ENUM ('Dog', 'Cat');
```

but the regression caused it to correspond to:

```
CREATE TYPE "PersonType" AS ENUM ('Dog', 'Cat');
```

This does not only affect newly created enums: we diff the wrong enums,
causing bad migrations for existing users of Migrate that have the enums
with the correct (mapped) names in their database.

Users reported the regression in prisma/prisma#16180
This commit should completely fix the problem.

The regression was introduced in an unrelated refactoring in
b6400f1. Enum names in column types
were replace with enum ids — a win for correctness. As part of this
refactoring, the `sql_schema_describer::Enum` type was made private, and
adding enums to a schema became possible only through
`SqlSchema::push_enum`. In the SQL migration connector

What could have caught this:

- Testing that the name of the database enum in the migration is the
  mapped name. We can test this:
    - Through `.assert_schema()`, which has existed for a long time.
      It is a gap in our test coverage that we weren't testing this.
    - The new single migration tests, like the test in this commit.
      They are new, and they are less work than the previous option.
- migrations-ci could have caught this, but it is currently neglected.

migration-engine/connectors/sql-migration-connector/src/sql_schema_calculator/sql_schema_calculator_flavour/postgres.rs

@@ -19,7 +19,7 @@ impl SqlSchemaCalculatorFlavour for PostgresFlavour {
             let sql_enum_id = ctx
                 .schema
                 .describer_schema
-                .push_enum(sql_namespace_id, prisma_enum.name().to_owned());
+                .push_enum(sql_namespace_id, prisma_enum.database_name().to_owned());
             ctx.enum_ids.insert(prisma_enum.id, sql_enum_id);
 
             for value in prisma_enum.values() {

migration-engine/migration-engine-tests/tests/single_migration_tests/postgres/enums/mapped_basic.prisma

@@ -0,0 +1,30 @@
+// tags=postgres
+// exclude=cockroachdb
+
+datasource pg {
+    provider = "postgresql"
+    url = env("TEST_DATABASE_URL")
+}
+
+model Avocado {
+  id String @id
+  status Ripeness
+}
+
+enum Ripeness {
+    NOT_RIPE_ENOUGH
+    TOO_RIPE
+
+    @@map("avocado_ripeness_status")
+}
+// Expected Migration:
+// -- CreateEnum
+// CREATE TYPE "avocado_ripeness_status" AS ENUM ('NOT_RIPE_ENOUGH', 'TOO_RIPE');
+// 
+// -- CreateTable
+// CREATE TABLE "Avocado" (
+//     "id" TEXT NOT NULL,
+//     "status" "avocado_ripeness_status" NOT NULL,
+// 
+//     CONSTRAINT "Avocado_pkey" PRIMARY KEY ("id")
+// );