[SG-656] Send a captcha bypass token back from the register endpoint (#2278)

* Send a captcha bypass token back from the register endpoint

* [review] Use existing user

* [review] Introduce ICaptcheProtectedResponseModel

src/Identity/Models/ICaptchaProtectedResponseModel.cs

@@ -0,0 +1,4 @@
+public interface ICaptchaProtectedResponseModel
+{
+    public string CaptchaBypassToken { get; set; }
+}

src/Identity/Models/RegisterResponseModel.cs

@@ -0,0 +1,14 @@
+using Bit.Core.Models.Api;
+
+namespace Bit.Identity.Models;
+
+public class RegisterResponseModel : ResponseModel, ICaptchaProtectedResponseModel
+{
+    public RegisterResponseModel(string captchaBypassToken)
+        : base("register")
+    {
+        CaptchaBypassToken = captchaBypassToken;
+    }
+
+    public string CaptchaBypassToken { get; set; }
+}