[ES|QL] Support the new RRF command

[stratoula]

Describe the feature:

ES has started adding support for a new RRF command elastic/elasticsearch#123396. This is the issue they use for tracking elastic/elasticsearch#123391.

We want to add this:

• ast parsing
• pretttifier etc
• autocomplete (the autocomplete is blocked till we move outside of snapshots - it seems to be extremely simple)

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)

[stratoula]

• RRF needs a licence to work
• You need to have _score, _id, _index into your metadata
• You need to have a fork

(_score might not be required though)

[drewdaemon]

@sddonne be sure to sync with me before you start on this one!