.NET: Add AIContextProvider support (#691)

* Add AIContextProvider support

* Address feedback.

* Address PR comments.

* Switch to valuetask and remove parallel calls for AIContextProvider

* Remove Model from ModelInvokingAsync method name

* Remove agent thread id again and remove it from context provider interface

* Add AIContextProvider serialization support to AgentThread and update sample to show this feature

* Address PR comments

* Improve memory sample

* Update sample comment.

* Remove AggregateAIContextProvider for now since it makes too many assumptions.  We can include it later as a sample if needed.

* Update AIContextProviders to have an Invoked method instead of MessagesAddingAsync.

* Remove unused using.

* Address PR comments.

* Address PR comment.

* Update comment.

* Update comment

* Address PR comments.

dotnet/src/LegacySupport/RequiredMemberAttribute/README.md

@@ -0,0 +1,9 @@
+Enables use of C# required members on older frameworks.
+
+To use this source in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectRequiredMemberOnLegacy>true</InjectRequiredMemberOnLegacy>
+</PropertyGroup>
+```

dotnet/src/LegacySupport/RequiredMemberAttribute/RequiredMemberAttribute.cs

@@ -0,0 +1,10 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.ComponentModel;
+
+namespace System.Runtime.CompilerServices;
+
+/// <summary>Specifies that a type has required members or that a member is required.</summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
+[EditorBrowsable(EditorBrowsableState.Never)]
+internal sealed class RequiredMemberAttribute : Attribute;