ES|QL FORK command

**What the feature is**

The `FORK` processing command creates multiple execution branches to operate on the same input data and combines the results in a single output table.

In this quarter, we aim to move `FORK` from tech preview to GA.

**Value proposition**

Users can run multiple queries, differentiate, mutate, and merge them together. This can enable such search use cases such as [hybrid search with custom result fusion](https://www.elastic.co/search-labs/blog/hybrid-search-multi-stage-retrieval-esql), useful for cases where you need to bring in results from different indices or retrieval types.

```
FROM books METADATA _score, _id, _index
| FORK (WHERE KNN(description_vector, ?query_vector) | SORT _score DESC | LIMIT 100)
(WHERE MATCH(description, ?query) | SORT _score DESC | LIMIT 100)
| FUSE // uses RRF by default
| SORT _score DESC
```

**Expected outcome**

We plan to make this General Availability.