Update Ecosystem.md with @clerk/fastify plugin (fastify#4571)

Author: dimkl

docs/Guides/Ecosystem.md

@@ -140,6 +140,8 @@ section.
 
 - [`@applicazza/fastify-nextjs`](https://github.com/applicazza/fastify-nextjs)
   Alternate Fastify and Next.js integration.
+- [`@clerk/fastify`](https://github.com/clerkinc/javascript/tree/main/packages/fastify)
+  Add authentication and user management to your Fastify application with Clerk.
 - [`@coobaha/typed-fastify`](https://github.com/Coobaha/typed-fastify) Strongly
   typed routes with a runtime validation using JSON schema generated from types.
 - [`@dnlup/fastify-doc`](https://github.com/dnlup/fastify-doc) A plugin for