Start milestone 3

- Add functionality to query online sources (Keyservers, etc) for
  certificates.
- Use introducers to determine which UserIDs should be fetched from
  online soources. Check certifications before importing certifiactes
  into the local cert store.
- start docker image for local end-to-end testing.

src/config.rs

@@ -14,6 +14,7 @@ pub struct HuskConfig {
     pub connection: String,
     pub sequoia_home: String,
     logfile_config: Option<String>,
+    pub keyservers: Option<Vec<String>>,
 }
 
 impl HuskConfig {
@@ -43,6 +44,15 @@ impl HuskConfig {
             },
         }
     }
+
+    pub fn get_keyservers(&self) -> Vec<String> {
+        if let Some(kss) = &self.keyservers {
+            kss.clone()
+        } else {
+            Vec::new()
+        }
+    }
+
 }
 
 pub type HuskConfigContainer = Arc<Mutex<HuskConfig>>;