source-field populated in aggregates with foreign refs (#38624)

* source-field populated in aggregates with foreign refs

When aggregates are populated that reference other tables, either a `:source-field` must be provided in the field metadata or a `:join-alias` needs to be provided. The former is preferred.

Prior to this PR, the first situation in the below simplified case could happen:

```clojure
;;Simplified case
;; busted
(qp/process-query
  {:database 1,
   ;; 59 is "PRODUCTS" -> "PRICE" which is not in the "REVIEWS" table
   :query    {:aggregation  [["sum" ["field" 59 {:base-type "type/Float"}]]]
              ;; "REVIEWS"
              :source-table 8},
   :type     "query"})

; Works
(qp/process-query
  {:database 1,
   ;; The `:source-field` metadata fixes the above query
   :query    {:aggregation  [["sum" ["field" 59 {:base-type    "type/Float"
                                                 ;; "REVIEWS" -> "PRODUCT_ID"
                                                 :source-field 71}]]]
              :source-table 8},
   :type     "query"})
```

This PR updates `metabase.automagic-dashboards.interestin/ground-metric` to use the `->reference` function for fields when building the `decoder` for transforming field names to references, which properly adds sources for external fields.

Fixes #38618

* Updating `grounded-metrics-test` to reflect updated field referencing.