Add Sensitive data type for passwords

As per title, password should also support Sensitive data type, trying to use it leads to:

```
Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Resource Statement, Collectd::Plugin::Network::Server[collectd.example.com]:
  parameter 'password' expects a value of type Undef or String, got Sensitive[String]
```