Emit ordinal output block for values aggregate

[dnhatn]

Time-series aggregations rely heavily on the values aggregation for collecting grouping values. For example:

TS k8s | STATS max(rate(request)) BY host

is translated to:

TS k8s
| STATS rate(request), VALUES(host) BY _tsid
| STATS max(`rate(request)`) BY host=`VALUES(host)`

We might change how these are executed later, but for now, we need to optimize the values aggregation for BytesRef, especially in cases with low cardinality. This change emits ordinal blocks as the output of the values aggregation, allowing the second aggregation to execute more efficiently. I will also open a PR to handle incoming ordinal blocks for the values aggregation.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

Hi @dnhatn, I've created a changelog YAML for you.

[dnhatn]

Thanks Kostas!