Specialize template base class for different script contexts

[apatrida]

For obvious ones such as:

 AGGS("aggs"), SEARCH("search"), UPDATE("update"), INGEST("ingest");

what might change for each? We can actually change them by the helper function being used, to make them more type safe and knowledgeable about what to expect for a given use.

[apatrida]

other field access patterns:
https://www.elastic.co/guide/en/elasticsearch/reference/5.6/modules-scripting-fields.html

[apatrida]

Where are _field values coming from, what script contexts?

[apatrida]

This can now be done more easily with 6.x compatibility, so this is a high priority now.