Support closure form of FetchService.autoGenCorrelationIds

[amcclain]

Currently spec'd as a boolean only, could be upgraded to Thunkable<boolean>, supporting a closure that would be passed FetchOptions for a given request to determine if an ID should be generated.

This could be used to e.g. only generate an ID for local/relative requests, avoiding doing so for CORS requests that don't support custom headers. Or for CORS requests to one API but not another. Or only for CORS requests - you get the idea. :)