[PM-5093][PM-7325] Added trial initiation email verification endpoint (#4221)

* Added trial initiation user verification endpoint

* Added explanatory comment for why we add artificial delay

* Updated RegistrationStart to Registration reference event

* Ensure that productTier query param is an int

* Added email value to trial initiation email

src/Core/Billing/Models/Api/Requests/Accounts/TrialSendVerificationEmailRequestModel.cs

@@ -0,0 +1,10 @@
+using Bit.Core.Auth.Models.Api.Request.Accounts;
+using Bit.Core.Billing.Enums;
+
+namespace Bit.Core.Billing.Models.Api.Requests.Accounts;
+
+public class TrialSendVerificationEmailRequestModel : RegisterSendVerificationEmailRequestModel
+{
+    public ProductTierType ProductTier { get; set; }
+    public IEnumerable<ProductType> Products { get; set; }
+}

src/Core/Billing/Models/Mail/TrialInititaionVerifyEmail.cs

@@ -0,0 +1,33 @@
+using Bit.Core.Auth.Models.Mail;
+using Bit.Core.Billing.Enums;
+
+namespace Bit.Core.Billing.Models.Mail;
+
+public class TrialInitiationVerifyEmail : RegisterVerifyEmail
+{
+    /// <summary>
+    /// See comment on <see cref="RegisterVerifyEmail"/>.<see cref="RegisterVerifyEmail.Url"/>
+    /// </summary>
+    public new string Url
+    {
+        get => $"{WebVaultUrl}/{Route}" +
+               $"?token={Token}" +
+               $"&email={Email}" +
+               $"&fromEmail=true" +
+               $"&productTier={(int)ProductTier}" +
+               $"&product={string.Join(",", Product.Select(p => (int)p))}";
+    }
+
+    public ProductTierType ProductTier { get; set; }
+
+    public IEnumerable<ProductType> Product { get; set; }
+
+    /// <summary>
+    /// Currently we only support one product type at a time, despite Product being a collection.
+    /// If we receive both PasswordManager and SecretsManager, we'll send the user to the PM trial route
+    /// </summary>
+    private string Route =>
+        Product.Any(p => p == ProductType.PasswordManager)
+            ? "trial-initiation"
+            : "secrets-manager-trial-initiation";
+}