Add multi columns non unique index and sql_nullable property. Important performance boost with relation map.

0.10.19

✨ Features

• Add support for multi-column non-unique IndexColumns in Meta.constraints #307
• Add sql_nullable field attribute that allows to set different nullable setting for pydantic model and for underlying sql column #308

🐛 Fixes

• Enable caching of relation map to increase performance #337
• Clarify and fix documentation in regard of nullable fields #339

💬 Other

• Bump supported databases version to <=5.2.