Add `signstar-config` crate to handle Signstar configs, administrative and non-administrative credentials on a Signstar host

Add signstar-config and accompanying integration tests.

nethsm/src/user.rs

 //! Module for credentials, user IDs and passphrases.
 
-use std::fmt::{Debug, Display};
+use std::fmt::Display;
 use std::str::FromStr;
 
 use nethsm_sdk_rs::apis::configuration::BasicAuth;
@@ -396,7 +396,7 @@ impl TryFrom<String> for UserId {
 /// Credentials for a [`NetHsm`][`crate::NetHsm`]
 ///
 /// Holds a user ID and an accompanying [`Passphrase`].
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct Credentials {
     pub user_id: UserId,
     pub passphrase: Option<Passphrase>,