insights: update search aggregation resolver with logic to determine if a search aggregation can be saved

Created by: chwarwick

The result of a search aggregation has a boolean SupportsPersistence which indicates if the search query is capable of being persisted as an insight on a dashboard. Currently this logic always returns false. Update this logic based on the criteria specified in RFC 727.

Additional a new field should be added to the resolver to indicate the reason that an aggregation is not able to be saved.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin