[ES|QL] Provide a better way to describe case fn signature

[dej611]

Describe the feature:

The case function signature is a bit of an exception as it has a true polymorphic structure poorly described in the current form:

• it requires at least 3 arguments, first of type boolean, the other 2 of any type
• if the 3rd argument is boolean, it can have further 2 args of any type
• for any 2 * N + 1th argument it applies the same concept as the previous step
  • basically if the else arg is a boolean the signature can be extended by other 2 args at the time.

Describe a specific use case for the feature: This signature logic needs to be defined in a serializable way and made it clear via the signature description, validation, autocomplete, etc...

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)