.Net: Update JsonSchemaMapper to incorporate the .NET 9 shim (#7960)

Updates `JsonSchemaMapper` to include the changes from
eiriktsarpalis/stj-schema-mapper#5. The changes
reimplement the component as a shim over the built-in schema exporter
available in System.Text.Json v9, for targets that support it. This
should give SK built-in schema generation for free simply by upgrading
its dependencies to STJ version 9.0.x once generally available.

cc @stephentoub @markwallace-microsoft

---------

Co-authored-by: Mark Wallace <127216156+markwallace-microsoft@users.noreply.github.com>

dotnet/src/InternalUtilities/src/Schema/JsonSchemaGenerationContext.cs

@@ -0,0 +1,98 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text.Json.Serialization.Metadata;
+
+namespace JsonSchemaMapper;
+
+    /// <summary>
+    /// Defines the context in which a JSON schema within a type graph is being generated.
+    /// </summary>
+#if EXPOSE_JSON_SCHEMA_MAPPER
+public
+#else
+internal
+#endif
+    readonly struct JsonSchemaGenerationContext
+{
+    internal JsonSchemaGenerationContext(
+        JsonTypeInfo typeInfo,
+        Type? declaringType,
+        JsonPropertyInfo? propertyInfo,
+        ParameterInfo? parameterInfo,
+        ICustomAttributeProvider? propertyAttributeProvider)
+    {
+        TypeInfo = typeInfo;
+        DeclaringType = declaringType;
+        PropertyInfo = propertyInfo;
+        ParameterInfo = parameterInfo;
+        PropertyAttributeProvider = propertyAttributeProvider;
+    }
+
+    /// <summary>
+    /// The <see cref="JsonTypeInfo"/> for the type being processed.
+    /// </summary>
+    public JsonTypeInfo TypeInfo { get; }
+
+    /// <summary>
+    /// The declaring type of the property or parameter being processed.
+    /// </summary>
+    public Type? DeclaringType { get; }
+
+    /// <summary>
+    /// The <see cref="JsonPropertyInfo"/> if the schema is being generated for a property.
+    /// </summary>
+    public JsonPropertyInfo? PropertyInfo { get; }
+
+    /// <summary>
+    /// The <see cref="System.Reflection.ParameterInfo"/> if a constructor parameter
+    /// has been associated with the accompanying <see cref="PropertyInfo"/>.
+    /// </summary>
+    public ParameterInfo? ParameterInfo { get; }
+
+    /// <summary>
+    /// The <see cref="ICustomAttributeProvider"/> corresponding to the property or field being processed.
+    /// </summary>
+    public ICustomAttributeProvider? PropertyAttributeProvider { get; }
+
+    /// <summary>
+    /// Checks if the type, property, or parameter has the specified attribute applied.
+    /// </summary>
+    /// <typeparam name="TAttribute">The type of the attribute to resolve.</typeparam>
+    /// <param name="inherit">Whether to look up the hierarchy chain for the inherited custom attribute.</param>
+    /// <returns>True if the attribute is defined by the current context.</returns>
+    public bool IsDefined<TAttribute>(bool inherit = false)
+        where TAttribute : Attribute =>
+        GetCustomAttributes(typeof(TAttribute), inherit).Any();
+
+    /// <summary>
+    /// Checks if the type, property, or parameter has the specified attribute applied.
+    /// </summary>
+    /// <typeparam name="TAttribute">The type of the attribute to resolve.</typeparam>
+    /// <param name="inherit">Whether to look up the hierarchy chain for the inherited custom attribute.</param>
+    /// <returns>The first attribute resolved from the current context, or null.</returns>
+    public TAttribute? GetAttribute<TAttribute>(bool inherit = false)
+        where TAttribute : Attribute =>
+        (TAttribute?)GetCustomAttributes(typeof(TAttribute), inherit).FirstOrDefault();
+
+    /// <summary>
+    /// Resolves any custom attributes that might have been applied to the type, property, or parameter.
+    /// </summary>
+    /// <param name="type">The attribute type to resolve.</param>
+    /// <param name="inherit">Whether to look up the hierarchy chain for the inherited custom attribute.</param>
+    /// <returns>An enumerable of all custom attributes defined by the context.</returns>
+    public IEnumerable<Attribute> GetCustomAttributes(Type type, bool inherit = false)
+    {
+        // Resolves attributes starting from the property, then the parameter, and finally the type itself.
+        return GetAttrs(PropertyAttributeProvider)
+            .Concat(GetAttrs(ParameterInfo))
+            .Concat(GetAttrs(TypeInfo.Type))
+            .Cast<Attribute>();
+
+        object[] GetAttrs(ICustomAttributeProvider? provider) =>
+            provider?.GetCustomAttributes(type, inherit) ?? Array.Empty<object>();
+    }
+}