Allow tables without primary key

[slavovthinks]

Primary keys are not required for Relational Table definition and are especially useless for join tables. Currently if you try to define a model without a primary key you get:

/lib/python3.10/site-packages/ormar/models/helpers/sqlalchemy.py", line 250, in populate_config_tablename_columns_and_pk
    raise ormar.ModelDefinitionError("Table has to have a primary key.")
ormar.exceptions.ModelDefinitionError: Table has to have a primary key.