Emit ordinal output block for values aggregate (#127201)

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.

Author: dnhatn

docs/changelog/127201.yaml

@@ -0,0 +1,5 @@
+pr: 127201
+summary: Emit ordinal output block for values aggregate
+area: ES|QL
+type: enhancement
+issues: []