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/husk_context.rs

@@ -26,6 +26,7 @@ pub struct HuskContext<'hc> {
     pub policy: StandardPolicy<'hc>,
     pub cert_store: CertStore<'hc>,
     pub local_trust_root: Cert,
+    pub signing_key: Option<Cert>,
     pub introducers: Vec<Introducer>,
     pub keyservers: Vec<String>,
     pub mail: MailContext<'hc>,
@@ -45,10 +46,19 @@ impl<'hc> HuskContext<'hc> {
         let local_trust_root = crypto::get_local_trust_root(&cert_store)?;
         let introducers = crypto::get_introducers(&cert_store, &policy, local_trust_root.fingerprint());
 
+        let signing_key = match crypto::get_signing_key(sequoia_home, &cert_store, &policy, &local_trust_root) {
+            Ok(Some(cert)) => Some(cert),
+            _ => {
+                log::warn!("No signing key found");
+                None
+            }
+        };
+
         Ok(HuskContext {
             policy,
             cert_store,
             local_trust_root,
+            signing_key,
             introducers,
             keyservers: config.get_keyservers(),
             mail: MailContext::new(),

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)
     }
 }