Schema creation and validation

[MasterMedo]

Memgraph is schemaless, but some use-cases require strict validation rules. Instead of enforcing the schema in Memgraph, the user should be able to enforce it easily in GQLAlchemy. With pydantic we should be able to add schema validation to the graph objects.

[MasterMedo]

This is completed, users can create their validation rules using the pydantic's Field function and their validators.