Add PolyTypeJsonFormatter as an experimental API

src/StreamJsonRpc/IGenericTypeArgAssist.cs

@@ -0,0 +1,42 @@
+namespace StreamJsonRpc;
+
+/// <summary>
+/// A non-generic interface with a generic method, for use with <see cref="IGenericTypeArgStore"/>
+/// so that generic contexts can be preserved and reused later in a non-generic context.
+/// </summary>
+internal interface IGenericTypeArgAssist
+{
+    /// <summary>
+    /// Invokes whatever functionality the implementation provides, using the specified generic type argument.
+    /// </summary>
+    /// <typeparam name="T">A generic type argument, whose semantics are up to the implementation.</typeparam>
+    /// <param name="state">Optional state that the caller may have provided.</param>
+    /// <returns>An arbitrary result, up to the implementation.</returns>
+    object? Invoke<T>(object? state = null);
+}
+
+/// <summary>
+/// A non-generic interface allowing invocation of a generic method with a type argument known only at runtime.
+/// </summary>
+/// <seealso cref="GenericTypeArgStore{T}"/>
+internal interface IGenericTypeArgStore
+{
+    /// <summary>
+    /// Invokes the generic <see cref="IGenericTypeArgAssist.Invoke{T}(object?)"/> method.
+    /// </summary>
+    /// <param name="assist">An implementation of <see cref="IGenericTypeArgAssist"/>.</param>
+    /// <param name="state">Optional state that the caller may have provided.</param>
+    /// <returns>An arbitrary result, up to the implementation.</returns>
+    object? Invoke(IGenericTypeArgAssist assist, object? state = null);
+}
+
+/// <summary>
+/// A generic implementation of <see cref="IGenericTypeArgStore"/>
+/// that can be created while in a generic context, so that a non-generic method can invoke another generic method later.
+/// </summary>
+/// <typeparam name="T">The generic type argument that must be supplied later.</typeparam>
+internal class GenericTypeArgStore<T> : IGenericTypeArgStore
+{
+    /// <inheritdoc/>
+    public object? Invoke(IGenericTypeArgAssist assist, object? state = null) => assist.Invoke<T>(state);
+}