Add Items (crud, models, endpoints), utils, refactor

[tiangolo]

• Update CRUD utils to use types better.
• Simplify Pydantic model names, from UserInCreate to UserCreate, etc.
• Upgrade packages.
• Add new generic "Items" models, crud utils, endpoints, and tests. To facilitate re-using them to create new functionality. As they are simple and generic (not like Users), it's easier to copy-paste and adapt them to each use case.
• Update endpoints/path operations to simplify code and use new utilities, prefix and tags in include_router.
• Update testing utils.
• Update linting rules, relax vulture to reduce false positives.
• Add full text search for items.
• Update project README.md with tips about how to start with backend.