Add support for PAGE_RERENDER_UPSERT

it might be interesting to ship PAGE_RERENDER_UPSERT from the producer in the cases where the https://gitlab.wikimedia.org/repos/search-platform/cirrus-rerender script is used to produce the initial rerender events. The script purpose is to refresh the index based on the MW api and thus in cases the page is missing from the search index the rerender event would end up being a noop. This patch adds a new option to list the reasons for which a rerender event might yield an upsert event rather than a normal rerender event. This is tested against the "reason" field of the rerender event stream.

Bug: T368010