feat(base): Implement Room::direct_targets_length.

This patch implements `Room::direct_targets_length`. It avoids to call
`Room::is_direct` if and only if we don't care about the room's state
and we don't want an async call, and if we don't want to pay the cost of
`Room::direct_targets` which clones the `HashSet` as an alternative way
to get a similar information than `Room::is_direct`.

crates/matrix-sdk-base/src/rooms/normal.rs

@@ -347,6 +347,12 @@ impl Room {
         self.inner.read().base_info.dm_targets.clone()
     }
 
+    /// If this room is a direct message, returns the number of members that
+    /// we're sharing the room with.
+    pub fn direct_targets_length(&self) -> usize {
+        self.inner.read().base_info.dm_targets.len()
+    }
+
     /// Is the room encrypted.
     pub fn is_encrypted(&self) -> bool {
         self.inner.read().is_encrypted()