www/authelia: enable secret mode to fetch secrets from files

Authelia provides the functionality using go templates so screts must
not be part of the config file, but can be read from external files.

That makes handling of keys more more convinient, like:

identity_providers:
  oidc:
    jwks:
      - key: {{ secret "/usr/local/etc/authelia/private_ecdsa.pem" | mindent 10 "|" | msquote }}
      - key: {{ secret "/usr/local/etc/authelia/private_rsa.pem" | mindent 10 "|" | msquote }}

Authelia must be started for this with the template filter which that commit
adds.

PR: 295354
Approved by: yuri (maintainer, timeout)