engine.#LoadSecret support

Fixes #1305

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/tasks/loadsecret/loadsecret.cue

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+engine.#Plan & {
+	actions: {
+		image: engine.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
+
+		generate: engine.#Exec & {
+			input: image.output
+			args: ["sh", "-c", "echo test > /secret"]
+		}
+
+		load: engine.#LoadSecret & {
+			input: generate.output
+			path:  "/secret"
+		}
+
+		verify: engine.#Exec & {
+			input: image.output
+			mounts: secret: {
+				dest:     "/run/secrets/test"
+				contents: load.contents
+			}
+			args: [
+				"sh", "-c",
+				#"""
+					test "$(cat /run/secrets/test)" = "test"
+					"""#,
+			]
+		}
+	}
+}