Better support for `ENUM`s

[OliverHofkens]

SQLAlchemy offers great support for ENUM fields, mapping them from either a list of options or a Python Enum. OData-Query doesn't utilize this feature well enough, and currently just crashes if you try to use a non-existing ENUM member.

[OliverHofkens]

This is partially solved in SQLAlchemy by passing validate_strings=True to the ENUM definition, which will raise LookupError when an unknown enum member is used. So this might not be our concern.

One adjacent feature that would be cool is supporting OData's enum type, which uses syntax like status eq PostStatus'DRAFT'.