This allows generator implementations to define how generator data
about a redirect gets merged into the redirect destination. This
does not change any defaults, only modules that explicitly set a
merge policy will have a change in their results.
This functionality is initially used for the search api modules to
retain search positions within the final result set.
Bug: T92796
Change-Id: If2f49e0fc3176288c95e870240754ee320a6bf91