Support Sensitive strings

[deric]

This PR adds support secrets passed as Sensitive data type.

[ju5t]

@deric thanks for the PR! And sorry for the late reply. I have a few questions.

1. Could you add some details to the docs about this feature?
2. I think ldap_secret is missing an unwrap, or am I missing something?
3. What is the difference between a secret and a configvalue? The configvalue doesn't seem to be used?

[deric]

@ju5t

1. Sure, there's not much to document. Sensitive type passwords aren't printed in logs like plain-text password and passwords are stored in git as encrypted strings.
2. Yes, you're right. Sorry, I've overlooked that.
3. Powerdns::ConfigValue allows empty strings, integers, secrets, etc. while Powerdns::Secret is strictly non-empty string or Sensitive type

[ju5t]

@deric regarding the documentation, it helps if people know how to use this. Or perhaps, it should even be the default in our documentation. As it's a safer option. It's an awesome feature.

About ConfigValue's. I overlooked it's use, sorry.

[deric]

@ju5t I've fixed unwrapping ldap_secret and updated the docs.