Allow to use Enums in custom DB schema

[KrilleGH]

I have a legacy DB which is receiving an API including new tables in a separate DB schema. The new code must not touch the existing objects in the public schema.

So all models derive from

class SchemaSQLModel(SQLModel):
    __table_args__ = {
        'schema': 'api',
    }

I have some fields which use Enums:

class HeroType(enum.StrEnum):
    normal = enum.auto()
    super = enum.auto()

class Hero(SchemaSQLModel):
    type: HeroType

The DB is PostgreSQL and I'm using the native Enum type (CREATE TYPE). Also these types reside in the custom schema.

When creating a new Hero the generated SQL has explicit casts to the types of the columns. However, for the type column only the plain name of the type is generated. Of course that doesn't work, as the Enum type isn't in the public schema.

The solution is to pass inherit_schema=True when instantiating SQLAlchemy's Enum class. I achived this by making a subclass and using that in get_sqlalchemy_type() instead of the plain one.

I'm aware this PR may not be in it's final form. Please advise what needs to be done to get it merged.