Add ability to override/replace authorizers.xml template

[juldrixx]

From nifikop created by michael81877: Orange-OpenSource/nifikop#169

Feature Request

Is your feature request related to a problem? Please describe.
Add the ability to override/replace the default authorizers.xml template so that you can include custom authorizers, userGroupProviders, and accessPolicyProviders.

Describe the solution you'd like to see

In authorizers.go it's clear that the authorizers.xml template is there for dynamic initial admin purposes, so i think it'd be wise to continue leveraging that template. Perhaps as an extension, nifikop could allow users to override that template to provide additional authorizers, userGroupProviders, and accessPolicyProviders.

Describe alternatives you've considered

It's not currently possible to configure any other authorizers.