lib/logstorage: support sort by (...) for the values stats function

[niladrix719]

Fixes #1426

Add sort by (...) support (with optional limit N for top-N selection) to the values stats function, mirroring json_values, so collected values can be ordered by fields of the matching log entries

Example

Ingest 3 logs:

curl -s -X POST 'http://localhost:9428/insert/jsonline' \
  -H 'Content-Type: application/stream+json' --data-binary '
{"_msg":"x","_time":"2024-01-01T00:00:03Z","ip":"10.0.0.3"}
{"_msg":"x","_time":"2024-01-01T00:00:01Z","ip":"10.0.0.1"}
{"_msg":"x","_time":"2024-01-01T00:00:02Z","ip":"10.0.0.2"}
'

Collect ip values ordered by _time:

curl -s 'http://localhost:9428/select/logsql/query' \
  --data-urlencode 'query=* | stats values(ip) sort by (_time) as ips'

{"ips":"[\"10.0.0.1\",\"10.0.0.2\",\"10.0.0.3\"]"}

Keep the 2 most recent with sort by (_time desc) limit 2:

curl -s 'http://localhost:9428/select/logsql/query' \
  --data-urlencode 'query=* | stats values(ip) sort by (_time desc) limit 2 as ips'

{"ips":"[\"10.0.0.3\",\"10.0.0.2\"]"}