.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/CompilerFeatureRequiredAttribute/CompilerFeatureRequiredAttribute.cs

@@ -0,0 +1,37 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+#pragma warning disable SA1623 // Property summary documentation should match accessors
+
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Indicates that compiler support for a particular feature is required for the location where this attribute is applied.
+/// </summary>
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+internal sealed class CompilerFeatureRequiredAttribute : Attribute
+{
+    public CompilerFeatureRequiredAttribute(string featureName)
+    {
+        FeatureName = featureName;
+    }
+
+    /// <summary>
+    /// The name of the compiler feature.
+    /// </summary>
+    public string FeatureName { get; }
+
+    /// <summary>
+    /// If true, the compiler can choose to allow access to the location where this attribute is applied if it does not understand <see cref="FeatureName"/>.
+    /// </summary>
+    public bool IsOptional { get; init; }
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the ref structs C# feature.
+    /// </summary>
+    public const string RefStructs = nameof(RefStructs);
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the required members C# feature.
+    /// </summary>
+    public const string RequiredMembers = nameof(RequiredMembers);
+}

dotnet/src/LegacySupport/CompilerFeatureRequiredAttribute/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>
+  <InjectCompilerFeatureRequiredOnLegacy>true</InjectCompilerFeatureRequiredOnLegacy>
+</PropertyGroup>
+```