feat: Add a base trust manager class

lib/src/trust/always.dart

@@ -0,0 +1,11 @@
+import 'package:meta/meta.dart';
+import 'package:omemo_dart/src/trust/base.dart';
+
+/// Only use for testing!
+/// An implementation of TrustManager that always trusts every device and thus
+/// has no internal state.
+@visibleForTesting
+class AlwaysTrustingTrustManager extends TrustManager {
+  @override
+  Future<bool> isTrusted(String jid, int deviceId) async => true;
+}

lib/src/trust/base.dart

@@ -0,0 +1,7 @@
+/// The base class for managing trust in OMEMO sessions.
+// ignore: one_member_abstracts
+abstract class TrustManager {
+  /// Return true when the device with id [deviceId] of Jid [jid] is trusted, i.e. if an
+  /// encrypted message should be sent to this device. If not, return false.
+  Future<bool> isTrusted(String jid, int deviceId);
+}