Cloud API: expose directory and file customizations

Expose the Directory and File customizations in the Cloud API. Also
validate the provided customizations while processing the compose
request by trying to convert them to internal representations
`fsnode.File` and `fsnode.Directory`.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

internal/cloudapi/v2/v2_test.go

@@ -927,7 +927,48 @@ func TestComposeCustomizations(t *testing.T) {
 				"disabled": [
 					"firewalld"
 				]
-			}
+			},
+			"directories": [
+				{
+					"path": "/etc/my/dir",
+					"mode": "0700"
+				},
+				{
+					"path": "/etc/my/dir1",
+					"mode": "0700",
+					"user": "user1",
+					"group": "user1",
+					"ensure_parents": true
+				},
+				{
+					"path": "/etc/my/dir2",
+					"mode": "0700",
+					"user": 1000,
+					"group": 1000,
+					"ensure_parents": true
+				}
+			],
+			"files": [
+				{
+					"path": "/etc/my/dir/file",
+					"mode": "0600",
+					"data": "Hello world!"
+				},
+				{
+					"path": "/etc/my/dir/file2",
+					"mode": "0600",
+					"user": "user1",
+					"group": "user1",
+					"data": "Hello world!"
+				},
+				{
+					"path": "/etc/my/dir/file3",
+					"mode": "0600",
+					"user": 1000,
+					"group": 1000,
+					"data": "Hello world!"
+				}
+			]
 		},
 		"image_request":{
 			"architecture": "%s",