feat: Allow default values in client: env (#2122)

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

docs/tests/core-concepts/client/plans/env.cue

@@ -1,11 +1,22 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"universe.dagger.io/docker"
+)
+
 dagger.#Plan & {
 	client: env: {
-		REGISTRY_USER:  string
+		// load as a string
+		REGISTRY_USER: string
+		// load as a secret
 		REGISTRY_TOKEN: dagger.#Secret
+		// load as a string, using a default if not defined
+		BASE_IMAGE: string | *"registry.example.com/image"
 	}
 
 	actions: pull: docker.#Pull & {
-		source: "registry.example.com/image"
+		source: client.env.BASE_IMAGE
 		auth: {
 			username: client.env.REGISTRY_USER
 			secret:   client.env.REGISTRY_TOKEN