crypto: withhold outgoing messages to unsigned dehydrated devices

Per #4313, we should not
send outgoing messages to dehydrated devices that are not signed by the current
pinned/verified identity.

Author: richvdh

crates/matrix-sdk-crypto/src/identities/device.rs

@@ -481,7 +481,7 @@ impl Device {
 
     /// Whether or not the device is a dehydrated device.
     pub fn is_dehydrated(&self) -> bool {
-        self.inner.device_keys.dehydrated.unwrap_or(false)
+        self.inner.is_dehydrated()
     }
 }
 
@@ -966,6 +966,11 @@ impl DeviceData {
     pub fn first_time_seen_ts(&self) -> MilliSecondsSinceUnixEpoch {
         self.first_time_seen_ts
     }
+
+    /// True if this device is an MSC3814 dehydrated device.
+    pub fn is_dehydrated(&self) -> bool {
+        self.device_keys.dehydrated.unwrap_or(false)
+    }
 }
 
 impl TryFrom<&DeviceKeys> for DeviceData {