Add API key authentication for Elsa client

Introduced an `ApiKeyHttpMessageHandler` to use an API key as the authorization header for Elsa API client. This ensures secure interaction with the Elsa server. Also made updates to the `ElsaClientOptions` class to include the API key. An `AddElsaClient` method is added with the API key parameter in `DependencyInjectionExtensions` class for convenience.

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

@@ -2,6 +2,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using Elsa.Api.Client.Contracts;
 using Elsa.Api.Client.Converters;
+using Elsa.Api.Client.HttpMessageHandlers;
 using Elsa.Api.Client.Options;
 using Elsa.Api.Client.Resources.ActivityDescriptorOptions.Contracts;
 using Elsa.Api.Client.Resources.ActivityDescriptors.Contracts;
@@ -31,6 +32,31 @@ namespace Elsa.Api.Client.Extensions;
 [PublicAPI]
 public static class DependencyInjectionExtensions
 {
+    /// <summary>
+    /// Adds the Elsa client to the service collection.
+    /// </summary>
+    /// <param name="services">The service collection.</param>
+    /// <param name="baseAddress">The base address of the Elsa API.</param>
+    /// <param name="apiKey">The API key to use for authentication.</param>
+    /// <param name="configureOptions">An optional delegate that can be used to configure the client options.</param>
+    /// <param name="configureBuilderOptions">An optional delegate that can be used to configure the client builder options.</param>
+    public static IServiceCollection AddElsaClient(this IServiceCollection services, Uri baseAddress, string apiKey, Action<ElsaClientOptions>? configureOptions = default, Action<ElsaClientBuilderOptions>? configureBuilderOptions = default)
+    {
+        services.AddScoped<ApiKeyHttpMessageHandler>();
+        return services.AddElsaClient(
+            options =>
+            {
+                options.BaseAddress = baseAddress;
+                options.ApiKey = apiKey;
+                configureOptions?.Invoke(options);
+            },
+            configureBuilderOptions: options =>
+            {
+                options.ConfigureHttpClientBuilder = builder => builder.AddHttpMessageHandler<ApiKeyHttpMessageHandler>();
+                configureBuilderOptions?.Invoke(options);
+            });
+    }
+    
     /// <summary>
     /// Adds the Elsa client to the service collection.
     /// </summary>

src/clients/Elsa.Api.Client/HttpMessageHandlers/ApiKeyHttpMessageHandler.cs

@@ -0,0 +1,30 @@
+using System.Net.Http.Headers;
+using Elsa.Api.Client.Options;
+using Microsoft.Extensions.Options;
+
+namespace Elsa.Api.Client.HttpMessageHandlers;
+
+/// <summary>
+/// An <see cref="HttpMessageHandler"/> that configures the outgoing HTTP request to use an API key as the authorization header.
+/// </summary>
+public class ApiKeyHttpMessageHandler : DelegatingHandler
+{
+    private readonly ElsaClientOptions _options;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ApiKeyHttpMessageHandler"/> class.
+    /// </summary>
+    public ApiKeyHttpMessageHandler(IOptions<ElsaClientOptions> options)
+    {
+        _options = options.Value;
+    }
+
+    /// <inheritdoc />
+    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        var apiKey = _options.ApiKey;
+        request.Headers.Authorization = new AuthenticationHeaderValue("ApiKey", apiKey);
+
+        return await base.SendAsync(request, cancellationToken);
+    }
+}

src/clients/Elsa.Api.Client/Options/ElsaClientOptions.cs

@@ -9,11 +9,11 @@ public class ElsaClientOptions
     /// Gets or sets the base address of the Elsa server.
     /// </summary>
     public Uri BaseAddress { get; set; } = default!;
-    
+
     /// <summary>
-    /// Gets or sets the API key to use when authenticating with the Elsa server.
+    /// Gets or sets the API key function to use when authenticating with the Elsa server.
     /// </summary>
-    public string ApiKey { get; set; } = default!;
+    public string? ApiKey { get; set; }
     
     /// <summary>
     /// Gets or sets a delegate that can be used to configure the HTTP client.