Fix doc page not updated on plugin activation

How to reproduce:
- Launch the Cat
- Goto /docs page
- Activate a plugin with a custom endpoint
- The custom endpoint can be used
- Refresh page /docs
- The new custom endpoint is not there

Why:
The openapi schema is cached by get_openapi_configuration_function

Solution:
Each time a function is decorated with `endpoint`, we flush the cache

core/cat/mad_hatter/decorators/endpoint.py

@@ -60,6 +60,7 @@ def _make_endpoint(endpoint):
             )
 
             cls.cheshire_cat_api.include_router(plugins_router, prefix=prefix)
+            cls.cheshire_cat_api.openapi_schema = None # Flush the cache of openapi schema
 
             return custom_endpoint