Add ConfigController to expose annotation config via API

Introduced a secured ConfigController with a GET endpoint at /api/Config/Annotations to provide annotation configuration data to clients. Utilizes dependency injection for configuration and includes necessary using directives.

EnvelopeGenerator.GeneratorAPI/Controllers/ConfigController.cs

@@ -0,0 +1,29 @@
+using EnvelopeGenerator.GeneratorAPI.Models.Annotation;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Options;
+
+namespace EnvelopeGenerator.GeneratorAPI.Controllers;
+
+/// <summary>
+/// Exposes configuration data required by the client applications.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of <see cref="ConfigController"/>.
+/// </remarks>
+[Route("api/[controller]")]
+[ApiController]
+[Authorize]
+public class ConfigController(IOptionsMonitor<AnnotationParams> annotationParamsOptions) : ControllerBase
+{
+    private readonly AnnotationParams _annotationParams = annotationParamsOptions.CurrentValue;
+
+    /// <summary>
+    /// Returns annotation configuration that was previously rendered by MVC.
+    /// </summary>
+    [HttpGet("Annotations")]
+    public IActionResult GetAnnotationParams()
+    {
+        return Ok(_annotationParams.AnnotationJSObject);
+    }
+}