EQL: Support for multi-value fields

[jrodewig]

Our support for array values (AKA multi-value fields) in EQL functions is not consistent:

• The startsWith and endsWith functions uses only the first array item.
• The wildcard function uses only the last array item.

Proposal

We should consider not supporting array values in EQL at all, similar to SQL. At the least, this could be documented.

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/EQL)

[costin]

Raised #55341 (and #55340 for startsWith).

[jrodewig]

We now document that multi-fields are not supported: https://www.elastic.co/guide/en/elasticsearch/reference/master/eql-syntax.html#eql-array-fields