Introduce an optional k param for knn query
If k is not set, knn query has the previous behaviour:
- `num_candidates` docs is collected from each shard. This `num_candidates` docs
are used for combining with results with other queries and aggregations on each shard.
- docs from all shards are merged to produce the top global `size` results
If k is set, the behaviour instead is following:
- `k` docs is collected from each shard. This `k` docs are used for
combining results with other queries and aggregations on each shard.
- similarly, docs from all shards are merged to produce the top global `size`
results.
Having `k` param makes it more intuitive for users to address their needs.
They also don't need to care and can skip `num_candidates` param for this query
as it is of more internal details to tune how knn search operates.
Closes#108473
