Begins adding support for running "tagged queries" to the compute
engine. Here, it's just the `LuceneSourceOperator` because that's
useful and contained.
Example time! Say you are running:
```
FROM foo
| STATS MAX(v) BY ROUND_TO(g, 0, 100, 1000, 100000)
```
It's *often* faster to run this as four queries:
* The docs that round to `0`
* The docs that round to `100`
* The docs that round to `1000`
* The docs that round to `100000`
This creates an ESQL operator that can run these queries, one after the
other and attach those tags.
Aggs uses this trick and it's *way* faster when it can push down count
queries, but it's still faster when it pushes doc loading things. This
implementation in `LuceneSourceOperator` is quite similar to the doc
loading version in _search.
I don't have performance measurements yet because I haven't plugged this
into the language. In _search we call this `filter-by-filter` and enable
it when each group averages to more than 5000 documents and when there
isn't an `_doc_count` field. It's faster in those cases not to push. I
expect we'll be pretty similar.
1b151eda4b