input: support yaml and json inputs

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

tests/cli/input/artifact/main.cue

@@ -0,0 +1,25 @@
+package testing
+
+import (
+	"dagger.io/dagger/op"
+	"dagger.io/dagger"
+)
+
+source: dagger.#Artifact
+foo:    "bar"
+
+bar: {
+	string
+
+	#up: [
+		op.#FetchContainer & {ref: "busybox"},
+		op.#Exec & {
+			args: ["cp", "/source/testfile", "/out"]
+			mount: "/source": from: source
+		},
+		op.#Export & {
+			format: "string"
+			source: "/out"
+		},
+	]
+}

tests/cli/input/artifact/testdata/testfile

@@ -0,0 +1 @@
+thisisatest