Configure saml via plugin UI

Currently the plugin relies on the existing `config.php` and
`authsources.php` files which are to be configured externally.

This commit add the ability for this plugin to generate the configuration
files itself using configuration params set from the phplist UI.

Signed-off-by: Fon E. Noel NFEBE <fenn25.fn@gmail.com>

Author: nfebe