feat: define StreamStore/ConsumerStore interfaces from Go store.go

Port IStreamStore, IConsumerStore, StoreMsg, StreamState, SimpleState,
ConsumerState, FileStoreConfig, StoreCipher, StoreCompression types.
Rename Models.StreamState → ApiStreamState to avoid namespace conflict.

src/NATS.Server/JetStream/Storage/MemStore.cs

@@ -132,11 +132,11 @@ public sealed class MemStore : IStreamStore
         }
     }
 
-    public ValueTask<StreamState> GetStateAsync(CancellationToken ct)
+    public ValueTask<ApiStreamState> GetStateAsync(CancellationToken ct)
     {
         lock (_gate)
         {
-            return ValueTask.FromResult(new StreamState
+            return ValueTask.FromResult(new ApiStreamState
             {
                 Messages = (ulong)_messages.Count,
                 FirstSeq = _messages.Count == 0 ? 0UL : _messages.Keys.Min(),