Support typehint overrides for column migrations

[piercefreeman]

We default our python types to the Postgres standard types (python int to postgres integer). But on the postgres side these values are only i32 by default. BIGINT is required as an explicit type definition to unlock the 64bit values. We add support for explicitly selecting the decorator type:

class TestModel(TableBase):
        id: int = Field(primary_key=True)
        # This should be BIGINT instead of INTEGER due to explicit_type
        big_number: int = Field(explicit_type=ColumnType.BIGINT)
        # This should be TEXT instead of VARCHAR due to explicit_type
        long_text: str = Field(explicit_type=ColumnType.TEXT)
        # This should be JSONB instead of JSON due to explicit_type
        data: dict = Field(is_json=True, explicit_type=ColumnType.JSONB)
        # Normal field without explicit_type for comparison
        normal_field: str = Field()