This PR adds a hash function to models which contain a primary key relation.
We know that a primary key does indeed offer a unique identifier for the row, and a concat of the primary key fields can thus be used to create a unique string identifying the row.

A use case for this would be the following.
Consider a select on the join of 2 tables, A and B.
In the resulting sequence of tuples, multiple instances of a row of either A or B can be present, as they match multiple times on their counterpart.

The most pythonic way to then retrieve the unique instances / rows of either A or B in the result set would be as follows.

# Retrieving the joined tuples, where A and B have 1 to many relationships.
statement = select(A, B).join(B, A.id == B.id)
result: Sequence[tuple[A,B]] = session.exec(statement).all()

# Unique instances of A
unique_a: list[A] = list(set([a for a, b in result]))

# Unique instances of B
unique_b: list[B] = list(set([b for a, b in result]))

This is currently not possible as the SQLModel class, and the underlying pydantic BaseModel are unhashable.
However, I propose that a SQLModel with properly defined primary keys IS actually hashable, as the primary key defines a unique, hashable property.
This would be in line with the underlying database, as some hash is ofcourse also used in the backend to identify this uniqueness.