feat: implement raft election and term state

src/NATS.Server/Raft/RaftTermState.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftTermState
+{
+    public int CurrentTerm { get; set; }
+    public string? VotedFor { get; set; }
+}
+
+public enum RaftRole
+{
+    Follower,
+    Candidate,
+    Leader,
+}