Adding nuget.config + Small Async fix (#62)

* Async fix

* Fix warnings

dotnet/nuget.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />  
+  </packageSources>
+</configuration>

dotnet/src/Microsoft.Agents.Abstractions/Agent.cs

@@ -46,7 +46,7 @@ public abstract class Agent
     /// If an agent supports multiple thread types, this method should return the default thread
     /// type for the agent or whatever the agent was configured to use.
     /// </remarks>
-    public abstract AgentThread CreateThreadAsync();
+    public abstract Task<AgentThread> CreateThreadAsync();
 
     /// <summary>
     /// Run the agent with no message assuming that all required instructions are already provided to the agent or on the thread.

dotnet/tests/Microsoft.Agents.Abstractions.UnitTests/AgentTests.cs

@@ -259,7 +259,7 @@ public class AgentTests
                 cancellationToken);
         }
 
-        public override AgentThread CreateThreadAsync()
+        public override Task<AgentThread> CreateThreadAsync()
         {
             throw new System.NotImplementedException();
         }