feat: implement raft election and term state

src/NATS.Server/Raft/RaftRpcContracts.cs

@@ -0,0 +1,12 @@
+namespace NATS.Server.Raft;
+
+public sealed class VoteRequest
+{
+    public int Term { get; init; }
+    public string CandidateId { get; init; } = string.Empty;
+}
+
+public sealed class VoteResponse
+{
+    public bool Granted { get; init; }
+}