Add GetRawValue() to return raw JSON in JsonSerializedField. (#9)

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## Description
This pull request introduces an `GetRawValue()` method in the
`JsonSerializedField` class to directly return the raw JSON string. This
enhancement provides a more straightforward and efficient way to access
the underlying JSON without the need for additional deserialization.

## Motivation
The primary motivation for this change is to simplify the process of
working with JSON data, particularly in scenarios where the JSON is
deserialized, modified, and then re-serialized. In my case, I needed to
deserialize JSON into a `SitecoreLayoutResponseContent` object, apply
modifications, and then serialize it back to JSON. Previously, this
required a custom `JsonConverter` that accessed the `_json` field via
reflection. With this improvement, such workarounds are no longer
necessary, making the code cleaner and more maintainable.

src/Sitecore.AspNetCore.SDK.LayoutService.Client/Serialization/Fields/JsonSerializedField.cs

@@ -18,6 +18,15 @@ public class JsonSerializedField(JsonDocument doc)
 
     private readonly string _json = doc != null ? doc.RootElement.GetRawText() : throw new ArgumentNullException(nameof(doc));
 
+    /// <summary>
+    /// Gets the raw JSON string representation of the field.
+    /// </summary>
+    /// <returns>A string containing the raw JSON data.</returns>
+    public string GetRawValue()
+    {
+        return _json;
+    }
+
     /// <inheritdoc/>
     protected override object? HandleRead(Type type)
     {

tests/Sitecore.AspNetCore.SDK.LayoutService.Client.Tests/Serialization/Fields/JsonSerializedFieldTests.cs

@@ -119,6 +119,21 @@ public void TryRead_InvalidJson_ThrowsException()
         field.Should().BeNull();
     }
 
+    [Fact]
+    public void GetRawValue_ReturnsExpectedJson()
+    {
+        // Arrange
+        const string json = "{\"value\": 100}";
+        JsonDocument token = JsonDocument.Parse(json);
+        JsonSerializedField sut = new(token);
+
+        // Act
+        string result = sut.GetRawValue();
+
+        // Assert
+        result.Should().Be(json);
+    }
+
     private class ValueField<T> : IField
     {
         public required T Value { get; set; }