Consider switching sensitive setting to admin_setting_encryptedpassword

[timhunt]

In https://tracker.moodle.org/browse/MDL-65818, Sam created a new admin_setting_encryptedpassword, which can store settings encrypted in the database.

Given what this plugin is doing, there are some of its settings which are secrets which might be better stored using this, rather than admin_setting_configpasswordunmask.

For your consideration.