Add extension method to get workflow instance ID

This adds a new extension method in HttpResponseMessageExtensions class which retrieves the workflow instance ID from the response. It also includes a HeaderNames class to maintain the consistency of header names used by the Elsa API.

src/clients/Elsa.Api.Client/Extensions/HeaderExtensions.cs

@@ -0,0 +1,14 @@
+using Elsa.Api.Client.Shared;
+
+namespace Elsa.Api.Client.Extensions;
+
+/// <summary>
+/// Contains extension methods for the <see cref="HttpResponseMessage"/> class.
+/// </summary>
+public static class HttpResponseMessageExtensions
+{
+    /// <summary>
+    /// Gets the workflow instance ID from the response.
+    /// </summary>
+    public static string? GetWorkflowInstanceId(this HttpResponseMessage response) => response.Headers.TryGetValues(HeaderNames.WorkflowInstanceId, out var values) ? values.FirstOrDefault() : default;
+}

src/clients/Elsa.Api.Client/Shared/HeaderNames.cs

@@ -0,0 +1,12 @@
+namespace Elsa.Api.Client.Shared;
+
+/// <summary>
+/// Contains header names used by the Elsa API.
+/// </summary>
+public static class HeaderNames
+{
+    /// <summary>
+    /// The name of the header that contains the workflow instance ID.
+    /// </summary>
+    public const string WorkflowInstanceId = "x-elsa-workflow-instance-id";
+}