Add SearchIndexField::getEngineHints()

Add SearchIndexField::getEngineHints()

Allows search engine clients that implement custom definitions
to pass engine hints used at index time.
Hints are a way to fine tune the behavior of the search engine
when handling a particular field.
As of now this is introduced for CirrusSearch to let SearchIndexField
implementations to control how the noop script is configured.
The noop hint with CirrusSearch allows to (for example):
- ignore an update if a numeric value does not change for more than X%
- control the merge strategy of complex fields

Bug: T166589
Change-Id: Ia560e41d33013c30ac47e5a60543f8cb133e61fb