Add project files.

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

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+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);
+    }
+}