Scoring support in ES|QL

[tteofili]

This meta-issue covers the work needed to enable scoring when using running full-text queries in ES|QL.

High Level Vision: score is just a regular column, no magic

Scoring is enabled through a METADATA field, _score. For example,

FROM employees METADATA _score
  | WHERE title:"test"
  | SORT _score

• Scoring is explicit, opt-in ( via METADATA )
• By adding METADATA _score a score column is added to the table
• The _score column is “dynamic”, potentially any command could alter it
• WHERE .. MATCH will affect the score (in the same as an ES query would)
• No implicit sorting on score. Sorting can be achieved through existing commands, e.g. a | SORT _score

### Tasks
- [ ] enable scoring with METADATA _score: #113120
- [ ] retain the scores of a single full text operation with multiple full text functions/operators
- [ ] clarify valid _score overrides
- [ ] make score collector parameters configurable/inferrable
- [ ] clarify scoring behavior with subqueries
- [ ] test for scoring consistency across _search and ES|QL with METADATA _score

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)