Adds support for the `offset` parameter to the `date_histogram` source
of composite aggs. The `offset` parameter is supported by the normal
`date_histogram` aggregation and is useful for folks that need to
measure things from, say, 6am one day to 6am the next day.
This is implemented by creating a new `Rounding` that knows how to
handle offsets and delegates to other rounding implementations. That
implementation doesn't fully implement the `Rounding` contract, namely
`nextRoundingValue`. That method isn't used by composite aggs so I can't
be sure that any implementation that I add will be correct. I propose to
leave it throwing `UnsupportedOperationException` until I need it.
Closes#48757
1d8e51f89d