Non indexable policy: death letter index (#26952)

Adding functionality to death letter index instead of dropping events.

When there's a mapping conflict events are dropped, this commits allows one attempting to index it into a different index instead so it can be re-ingested later vs dropping or stalling ingest. The index currently must reside on the same ES cluster.

* buildCollectPublishFails and other functions changed to be a function of Client so not to have to pass arguments

Co-authored-by: Noémi Ványi <[email protected]>
(cherry picked from commit 60cf09d)