Optional argument default-fn to default-value-transformer

See an example usage here:

https://github.com/piotr-yuxuan/malli-cli/blob/main/src/piotr_yuxuan/malli_cli.clj#L220-L222

This doesn't change existing behaviour but only add `default-fn`
optional argument. When set, this function is applied on the default
value found under key `key`. This optional, additional indirection
allows a seamless way to define `:env-var` defaults.

src/malli/transform.cljc

@@ -378,7 +378,7 @@
 (defn default-value-transformer
   ([]
    (default-value-transformer nil))
-  ([{:keys [key defaults] :or {key :default}}]
+  ([{:keys [key default-fn defaults] :or {key :default, default-fn identity}}]
    (let [get-default (fn [schema]
                        (if-some [e (some-> schema m/properties (find key))]
                          (constantly (val e))
@@ -402,7 +402,9 @@
                                           (reduce-kv
                                             (fn [acc k f]
                                               (if-not (contains? x k)
-                                                (assoc acc k (f))
+                                                (if-let [d (default-fn (f))]
+                                                  (assoc acc k d)
+                                                  acc)
                                                 acc))
                                             x defaults)
                                           x)))))}]