Configure enabled/allowed authentication methods for gRPC frontend by listing them in config

Following the discussion in the ops meeting today, we agreed that we want to specify the allowed authentication methods on the cta frontend as a list of comma-separated values (accepted values are krb5, jwt).

In the case of XRootD/SSI we have this config entry:

# Only Kerberos 5 and sss are allowed
sec.protbind * only sss krb5