[ES|QL] RERANK command - Prevent unnamed fields

[afoucret]

Acceptance criteria:

• We should enforce the usage of an identifier for fields used in the RERANK command.

The following command:

FROM movies METADATA _score
  | WHERE QSTR("star wars")
  | RERANK "star wars" ON title, SUBSTRING(overview, 0, 100), actors WITH rerankerInferenceId

should fail because no name has been provided for the SUBSTRING expression.

The correct way to write it is:

FROM movies METADATA _score
  | WHERE QSTR("star wars")
  | RERANK "star wars" ON title, short_overview=SUBSTRING(overview, 0, 100), actors WITH rerankerInferenceId

[elasticsearchmachine]

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

[afoucret]

#127416 is merged and does implement the check