Allow stellar enrichments to be specified by a list as well as a map

Currently, stellar enrichments are specified by a map associating fields with the stellar expressions associated with the fields. There is a significant downside here in that you cannot update previously assigned fields. For instance, the following cannot be represented currently:

 "fieldMap": {
       ...
      "stellar" : {
        "config" : {
          "hostname" : "if ENDS_WITH(hostname, '.') then CHOP(hostname) else hostname",
          "hostname" : "TO_LOWER(hostname)"
        }
      }
    }

A consequent of this deficiency is that we also cannot use temporary variables and unset them after their use inside an enrichment group.

The proposed change is to allow users to use lists of strings representing stellar expression assignments with the same syntax as the Stellar REPL. This would be as an alternative to maps, but the map syntax would also be supported for legacy.