ES|QL: Add FUSE command

[ioanatia]

related: #123389

Adds the FUSE command.

Example:

FROM books METADATA _id, _index, _score
| FORK ( WHERE title:"Shakespeare" | SORT _score, _id DESC  )
       ( WHERE semantic_title:"Shakespear" | SORT _score, _id DESC)
| FUSE rrf OPTIONS {"weights": { "fork1": 0.3, "fork2": 0.7 } }
| SORT _score DESC, _id, _index

TODOs for this PR:

• analyzer/parser statement tests with full options
• validation of fuse options + tests for validation
• CSV tests: use FUSE with ROW, LOOKUP JOIN etc
• update operators describe (for profile)
• since we change MapExpression - make sure that functions using MapExpression are not going to throw funny errors if nested maps are passed as params.
• operator tests

followups that we will do separately:

• fuse should handle unsupported fields - we should at least test for it
• check that fuse can work with input containing all supported data types - since we use STATS, this means VALUES needs to support all data types.
• optimization step to remove FuseScoreEval when linear is used without score normalization and weights.
• fuse needs to be executed at the coordinator level since it needs the full input
• check that fuse can be used in FORK branches
• make sure the generative tests have a generator for FUSE
• issue warnings when the score or discriminator colums have null values in the operator

[ioanatia]

closing PR since we split the implementation into smaller and separate PRs.