DRAFT: Add the ability to run scripts to build fields at index time

[romseygeek]

External API: Field mapper definitions get an added 'script' parameter (currently implemented only for numerics):

"properties" : {
    "message_length" : {
        "type" : "long",
        "script" : "emit(doc['message'].length())"
    }
}

An input source containing a 'message_length' field throws an error at parse time.

Internal API: Mappers get a 'postParse()' method, which is called on the root mapper once all normal parsing is done and propagates through the mapper tree. Mappers with scripts execute them at this point. The scripts don't share any code with their runtime equivalents, but expose the same context - _source (just a source lookup built from the input document), emit() and doc. The latter is constructed using value fetchers, which results in some changes to this internal API: specifically, MappedFieldType.valueFetcher() now takes a function to expand source paths, rather than a SearchExecutionContext (because we have no SEC at index time); and ValueFetcher.fetchValues() now takes a ValuesLookup rather than just a SourceLookup, which includes field data as well.

[romseygeek]

I should point out that there are a couple of refactorings that we can do in separate parts here to prepare for the new functionality, which will make the final thing easier to review:

• Change the parameter onValueFetcher.fetchValues from SourceLookup to ValuesLookup
• Change the parameter on MappedFieldType.valueFetcher from SearchExecutionContext to Function<String, Set<String>>

[romseygeek]

Closing in favour of #69531