[DRAFT] Initial changes for full-text search in ES|QL

[ChrisHegarty]

DRAFT DRAFT DRAFT - not for review

Initial changes for full-text search in ES|QL.

For example:

SEARCH images [
  | WHERE date > now() - 1 month
  | RANK MATCH(scene, "mountain lake")
  | WHERE _score > 0.1
  | LIMIT 100
]
| STATS c = COUNT(votes) BY rating
| LIMIT 5

Non-exhaustive list of outstanding tasks:

• [ ] Review all interactions with current stable code
• [ ] Investigate CSV testing
• [ ] review and iterate on lexer and grammar
• [x] unit tests for operator
• [ ] flesh out more examples and tests
• [ ] test release and snapshot builds
• [x] fix issue with filter and match scoring 0.0
• [ ] examine the use of float for _score (encoded into an IntVector)
• [ ] topN on the coordinator
• [ ] float block, and float type BWC
• [x] support WHERE MATCH
• [ ] ...