[WIP] Allow Bucket Script aggregation to reference on string results

[Hohol]

Closes #36642

[Hohol]

@polyfractal, I experimented a bit to see if the issue can be solved without big refactoring. This solution worked for my simple use case. Probably, it'll break if you try to do anything else :)
What do you think about the idea?

[elasticmachine]

Pinging @elastic/es-analytics-geo

[polyfractal]

Hmm, need to think on this one. A potential concern is that this would allow bucket scripts to return strings (great!) but since none of the other pipelines or framework were adjusted, nothing else could use the generated values. E.g. a second bucket_script referencing this one would break because it expects a double.

It might be an acceptable middle ground if well documented, or maybe too user-unfriendly. Not sure, I'm going to mark this as team-discuss to see what the rest of the team thinks.

[RubieV]

This would work very well for us, in a use-case where we want to apply a script returning a category, of type String, based on both numeric values as well as string matches.