Enable inheriting methods from schemas

[ch-ahindura]

Is your feature request related to a problem? Please describe.
Many times I just want to define my methods on the schema and have them available to all model types.
Could we allow for that?

Example:

class Booking(BaseModel):
    id: str = Field()
    start: datetime = Field()
    end: datetime = Field()

    @computed_field
    @property
    def duration_secs(self) -> float:
        """The duration in seconds"""
        return (self.end - self.start).total_seconds()

SqlBooking = SQLModel("SqlBooking", Booking)