feat: add stream replica groups and leader stepdown

src/NATS.Server/Raft/RaftNode.cs

@@ -9,6 +9,7 @@ public sealed class RaftNode
 
     public string Id { get; }
     public int Term => TermState.CurrentTerm;
+    public bool IsLeader => Role == RaftRole.Leader;
     public RaftRole Role { get; private set; } = RaftRole.Follower;
     public RaftTermState TermState { get; } = new();
     public long AppliedIndex { get; set; }
@@ -99,6 +100,8 @@ public sealed class RaftNode
     public void RequestStepDown()
     {
         Role = RaftRole.Follower;
+        _votesReceived = 0;
+        TermState.VotedFor = null;
     }
 
     private void TryBecomeLeader(int clusterSize)