Start with signing outgoing mails

- Add a scheme on how to discover Husk signing keys. Siging keys mut be
  certified by the `local trust root` with a signature having a notation
  with a name equal to `signing_keys@husk-projet.org`.
- Fetch the key from the keystore.
- Expand `authenticate()` so that notation can be added to the
  signature.
- Add a test to create a signing key and fetch it from the keystore.

src/types/introducer.rs

@@ -83,7 +83,8 @@ impl Introducer {
             cert,
             crypto::Role::Introducer,
             true,
-            Some(domains))
+            Some(domains),
+            None)
     }
 
     /// Remove an introducer.
@@ -101,6 +102,7 @@ impl Introducer {
             cert,
             crypto::Role::Introducer,
             false,
+            None,
             None)
     }
 }