Use lenient resolver type (#956)

In #921, a stronger type applied to OperationHandlerOptions['resolver']
so that end users would have an idea of what the parameters are for
their custom resolvers. It went too far in stipulating a return type.
Set the return type to unknown and let users decide how much type safety
they need in their resolver.

Fixes #952

src/framework/types.ts

@@ -68,7 +68,7 @@ export type OperationHandlerOptions = {
     handlersPath: string,
     route: RouteMetadata,
     apiDoc: OpenAPIV3.Document,
-  ) => RequestHandler | Promise<RequestHandler>;
+  ) => unknown;
 };
 
 export type Format = {