Generate schema.sql from models.yml for relational database

[r-peschke]

To remember:

• [ ] Common test setup for meta and backend repo
• [ ] See connection handler and usage with psycopg3
• [ ] Read adapter
• [ ] Write adapter
• [ ] Add Required checks to 1tR:1Gt und 1tR:1GtR, tests für alle existierenden Required Varianten
• [ ] Extend validate and generate_models.py with implemented information for relation:relation
• [ ] Migrations to and in relational db
• [ ] Test for motion_identical
• [ ] Generation: on_delete
• [ ] Realization or not for more attributes: readonly (in action-payload), deferred, ..

[jsangmeister]

Please follow https://github.com/OpenSlides/OpenSlides/wiki/Development-organization#creating-issues--pull-requests