Avoid sa_column where possible

[jpmckinney]

It can cause programmer errors like in #235.

• DECIMAL: https://sqlmodel.tiangolo.com/advanced/decimal/#decimals-in-sqlmodel
• needed for DateTime(timezone=True): Dose there any better way to write timezone aware datetime field without using the SQLAlchemy ? fastapi/sqlmodel#539
• needed for JSON JSON and Array types for sqlmodel fastapi/sqlmodel#42 (<class 'dict'> has no matching SQLAlchemy type if sa_type=JSON is omitted)
• DateTime(timezone=False) (maybe better to stay consistent with DateTime(timezone=True))

[jpmckinney]

When making changes to Decimal in #238, we had changed the type annotation from Decimal to our custom type FLOAT_DECIMAL 56ea76a, based on this documentation: https://docs.pydantic.dev/latest/api/standard_library_types/#decimaldecimal

Relevant comments:

• Upgrade Pydantic, FastAPI, SQLModel #238 issue description
• Upgrade Pydantic, FastAPI, SQLModel #238 (comment)
• Upgrade Pydantic, FastAPI, SQLModel #238 (comment)