Allow adding digital signatures to webhook headers

- Because webhook bodies and headers are transformed separately,
  Handlebars cannot be used to generate digital signatures using both
  the body and the header. This change allows the user to specify two
  callbacks to use to generate a digital signature for the webhook
  before it is sent.

src/WireMock.Net.Abstractions/Models/IDigitalSignature.cs

@@ -0,0 +1,41 @@
+// Copyright © WireMock.Net
+
+using System.Threading.Tasks;
+using WireMock.Util;
+
+namespace WireMock.Models;
+
+/// <summary>
+/// A delegate encapsulating logic for fetching or generating a private key.
+/// </summary>
+/// <returns>An async task containing the value of the private key.</returns>
+public delegate Task<string> PrivateKeyCallback();
+
+/// <summary>
+/// A delegate encapsulating logic for fetching or generating a digital signature.
+/// </summary>
+/// <param name="privateKey">The value of the private key returned by <see cref="PrivateKeyCallback"/>.</param>
+/// <param name="bodyData">The body data associated with the request.</param>
+/// <returns>An async task containing the value of the digital signature.</returns>
+public delegate Task<string> DigitalSignatureCallback(string privateKey, IBodyData? bodyData);
+
+/// <summary>
+/// An interface encapsulating logic for generating a digital signature.
+/// </summary>
+public interface IDigitalSignature
+{
+    /// <summary>
+    /// The header name to use for the digital signature.
+    /// </summary>
+    string HeaderName { get; set; }
+
+    /// <summary>
+    /// The callback used to fetch or generate the private key.
+    /// </summary>
+    PrivateKeyCallback PrivateKeyCallback { get; set; }
+
+    /// <summary>
+    /// The callback used to fetch or generate the digital signature.
+    /// </summary>
+    DigitalSignatureCallback DigitalSignatureCallback { get; set; }
+}

src/WireMock.Net.Abstractions/Models/IWebhookRequest.cs

@@ -60,4 +60,14 @@ public interface IWebhookRequest
     /// Gets or sets the maximum random delay in milliseconds.
     /// </summary>
     int? MaximumRandomDelay { get; set; }
+
+    /// <summary>
+    /// Gets or sets the value used to determine if digital signatures are generated.
+    /// </summary>
+    bool? UseDigitalSignatures { get; set; }
+
+    /// <summary>
+    /// Gets or sets the values used generate digital signatures.
+    /// </summary>
+    IDigitalSignature[]? DigitalSignatures { get; set; }
 }

src/WireMock.Net/Http/WebhookSender.cs

@@ -78,6 +78,31 @@ IResponseMessage originalResponseMessage
             webhookRequestUrl = webhookRequest.Url;
         }
 
+        // Add digital signatures to header (if required)
+        if (webhookRequest.UseDigitalSignatures == true)
+        {
+            if (webhookRequest.DigitalSignatures is not null)
+            {
+                foreach (var digitalSignature in webhookRequest.DigitalSignatures)
+                {
+                    var privateKey = await digitalSignature.PrivateKeyCallback();
+                    var signature = await digitalSignature.DigitalSignatureCallback(privateKey, bodyData);
+
+                    // headers may be null here
+                    headers ??= new Dictionary<string, WireMockList<string>>();
+
+                    if (headers.ContainsKey(digitalSignature.HeaderName))
+                    {
+                        headers[digitalSignature.HeaderName].Add(signature);
+                    }
+                    else
+                    {
+                        headers[digitalSignature.HeaderName] = new WireMockList<string>(signature);
+                    }
+                }
+            }
+        }
+
         // Create RequestMessage
         var requestMessage = new RequestMessage(
             new UrlDetails(webhookRequestUrl),

src/WireMock.Net/Models/DigitalSignature.cs

@@ -0,0 +1,20 @@
+// Copyright © WireMock.Net
+
+using System.Threading.Tasks;
+
+namespace WireMock.Models;
+
+/// <summary>
+/// A class encapsulating logic for generating a digital signature.
+/// </summary>
+public class DigitalSignature : IDigitalSignature
+{
+    /// <inheritdoc/>
+    public string HeaderName { get; set; } = "";
+
+    /// <inheritdoc/>
+    public PrivateKeyCallback PrivateKeyCallback { get; set; } = async () => await Task.FromResult("");
+
+    /// <inheritdoc/>
+    public DigitalSignatureCallback DigitalSignatureCallback { get; set; } = async (privateKey, bodyData) => await Task.FromResult("");
+}

src/WireMock.Net/Models/WebhookRequest.cs

@@ -40,4 +40,10 @@ public class WebhookRequest : IWebhookRequest
 
     /// <inheritdoc />
     public int? MaximumRandomDelay { get; set; }
+
+    /// <inheritdoc />
+    public bool? UseDigitalSignatures { get; set; }
+
+    /// <inheritdoc />
+    public IDigitalSignature[]? DigitalSignatures { get; set; }
 }