Add MiddlewareProvider interface for extension middleware

Allows cloud extensions to inject middleware that wraps OSS routes
(e.g. billing enforcement) before they are registered.

pkg/cpextension/extension.go

@@ -5,6 +5,7 @@ package cpextension
 
 import (
 	"context"
+	"net/http"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -48,3 +49,10 @@ type Extension interface {
 	// Each function should start its own goroutine(s) and return.
 	BackgroundWorkers(ctx ServerContext) []func(context.Context)
 }
+
+// MiddlewareProvider is optionally implemented by extensions that need
+// middleware applied before OSS routes are registered. This allows
+// cloud middleware to wrap existing OSS routes (e.g. billing checks).
+type MiddlewareProvider interface {
+	Middlewares(ctx ServerContext) []func(http.Handler) http.Handler
+}