EQL: implement case sensitivity for between string function

[astefan]

Following a similar approach in #56404, implement case sensitivity based on request parameter for between string function.

[elasticmachine]

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

[costin]

It's worth considering whether we should still preserve the case sensitivity switch for between - iirc, its usage was not wide so maybe we could remove it altogether.

[costin]

Superseded by #62255