Add time_bucket field for TimescaleDB support

[moritz89]

TimescaleDB has the time_bucket field to take full advantage of their hypertables: https://docs.timescale.com/api/latest/hyperfunctions/time_bucket/

This could be supported with:

class TimeBucketField(MultiField):
    def __init__(self, bucket_seconds=60.0, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.bucket_seconds = float(bucket_seconds)
        self.auto_alias = f"{self.field.name}_bucket"

    def get_select_sql(self):
        return f"""time_bucket('{self.bucket_seconds:.3f}s',"{self.field.name}")"""

Would this be in the scope of the project?