Adding index settings for es812PostingsFormat #137855
Description
Today this used if index.mode is either time_series or logsdb. I like to introduce a new setting for this that default to true if index.mode is logsdb or time_series, but does allow someone to overwrite in case of other index modes. Note es812PostingsFormat was default for all index modes before 9.1.
The logic that controls which postings format gets used is in PerFieldFormatSupplier constructor a new index setting can be added to IndexSettings class.