Databackend-agnostic vector-types

[blythed]

In embedding models, we have this type of logic:

    def _pre_create(self, db):
        """Pre creates the model.

        If the datatype is not set and the datalayer is an IbisDataBackend,
        the datatype is set to ``sqlvector`` or ``vector``.

        :param db: The datalayer instance.
        """
        if self.datatype is not None:
            return

        from superduper.components.vector_index import sqlvector, vector

        if db.databackend.db_type == DBType.SQL:
            self.datatype = sqlvector(shape=self.shape)
        else:
            self.datatype = vector(shape=self.shape)

We need to find a way to do this more generally so that apps with vectors can be deployed on all databackends, without editing.