Add MultiPart/MimePart Request Matcher (#981)

* wip

* .

* mm

* x

* .

* .

* .

* tests

* .

* more tests

* trans

* x

* win

* fix

* .

* tests

src/WireMock.Net/Util/MimeKitUtils.cs

@@ -0,0 +1,42 @@
+#if MIMEKIT
+using System;
+using System.IO;
+using System.Text;
+using MimeKit;
+using WireMock.Http;
+using WireMock.Types;
+
+namespace WireMock.Util;
+
+internal static class MimeKitUtils
+{
+    public static MimeMessage GetMimeMessage(IBodyData? bodyData, string contentTypeHeaderValue)
+    {
+        var bytes = bodyData?.DetectedBodyType switch
+        {
+            // If the body is bytes, use the BodyAsBytes to match on.
+            BodyType.Bytes => bodyData.BodyAsBytes!,
+
+            // If the body is a String or MultiPart, use the BodyAsString to match on.
+            BodyType.String or BodyType.MultiPart => Encoding.UTF8.GetBytes(bodyData.BodyAsString!),
+
+            _ => throw new NotSupportedException()
+        };
+
+        var fixedBytes = FixBytes(bytes, contentTypeHeaderValue);
+        return MimeMessage.Load(new MemoryStream(fixedBytes));
+    }
+
+    private static byte[] FixBytes(byte[] bytes, WireMockList<string> contentType)
+    {
+        var contentTypeBytes = Encoding.UTF8.GetBytes($"{HttpKnownHeaderNames.ContentType}: {contentType}\r\n\r\n");
+
+        var result = new byte[contentTypeBytes.Length + bytes.Length];
+
+        Buffer.BlockCopy(contentTypeBytes, 0, result, 0, contentTypeBytes.Length);
+        Buffer.BlockCopy(bytes, 0, result, contentTypeBytes.Length, bytes.Length);
+
+        return result;
+    }
+}
+#endif