Add Postgres specific sample class for JSON queries

- Introduced PostgresSpecificSample.cs file.
- Implemented a sample class using ITestOutputHelper.
- Added ToJsonQuery method to generate and test SQL queries.

Author: mk3008

demo/Sample/PostgresSpecificSample.cs

@@ -0,0 +1,55 @@
+using Carbunqlex;
+using Xunit.Abstractions;
+
+namespace Sample;
+
+/// <summary>
+/// Sample for Postgres specific features.
+/// </summary>
+/// <param name="output"></param>
+public class PostgresSpecificSample(ITestOutputHelper output)
+{
+    private readonly ITestOutputHelper output = output;
+
+    /// <summary>
+    /// Sample to generate a query that returns JSON
+    /// </summary>
+    [Fact]
+    public void ToJsonQuery()
+    {
+        var query = QueryAstParser.Parse("""  
+          select
+              posts.post_id
+              , posts.title
+              , posts.content
+              , posts.created_at
+              , users.user_id
+              , users.name as user_name
+              , blogs.blog_id
+              , blogs.name as blog_name
+              , organizations.organization_id
+              , organizations.name as organization_name
+          from
+              posts
+              inner join users on posts.user_id = users.user_id
+              inner join blogs on posts.blog_id = blogs.blog_id
+              inner join organizations on blogs.organization_id = organizations.organization_id
+          where
+              posts.post_id = :post_id
+          """);
+
+        query.NormalizeSelectClause()
+            .Serialize("organizations", objectName: "organization")
+            .Serialize("users", objectName: "user")
+            .Serialize("blogs", objectName: "blog", include: ["organization"])
+            .Serialize("posts", objectName: "post", include: ["user", "blog"])
+            .ToJson();
+
+        var actual = query.ToSql();
+
+        output.WriteLine(actual);
+
+        var expected = "select row_to_json(d) from (select json_build_object('post_id', posts.post_id, 'title', posts.title, 'content', posts.content, 'created_at', posts.created_at, 'user', json_build_object('user_id', users.user_id, 'user_name', users.name), 'blog', json_build_object('blog_id', blogs.blog_id, 'blog_name', blogs.name, 'organization', json_build_object('organization_id', organizations.organization_id, 'organization_name', organizations.name))) as post from posts inner join users on posts.user_id = users.user_id inner join blogs on posts.blog_id = blogs.blog_id inner join organizations on blogs.organization_id = organizations.organization_id where posts.post_id = :post_id) as d limit 1";
+        Assert.Equal(expected, actual);
+    }
+}