chore: add Caddy config to ServiceSpec, load x-caddy to it

pkg/api/caddy.go

@@ -0,0 +1,8 @@
+package api
+
+// CaddySpec is the Caddy reverse proxy configuration for a service.
+type CaddySpec struct {
+	// Config contains the Caddy config (Caddyfile) content. It must not conflict with the Caddy configs
+	// of other services.
+	Config string
+}