EQL: implement case sensitivity for stringContains function

[astefan]

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

[elasticmachine]

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

[costin]

this requires a bit of thinking as they both rely on regex and supporting case-sensitivity means a new regex. Until the wildcard situation gets resolved, at least the infrastructure in the classes can be hooked up.