fix(authelia): add authn_strategies for Beszel forward-auth compatibility (#76)

Fixes recurring authentication prompts when using Authelia v4.39.15+ with Beszel by adding CookieSession authentication strategy to the forward-auth endpoint configuration.

References:
- https://beszel.dev/guide/common-issues#authelia-forward-auth-not-working
- henrygd/beszel#1482

Author: Vidhin05

apps/authelia/config/configuration.yml

@@ -105,6 +105,8 @@ server:
     authz:
       forward-auth:
         implementation: 'ForwardAuth'
+        authn_strategies:
+          - name: 'CookieSession'
         # authn_strategies: []
       # ext-authz:
         # implementation: 'ExtAuthz'