[PM-20084] [PM-20086] Add TrialLength parameter to trial initiation endpoint and email (#5770)

* Add trial length parameter to trial initiation endpoint and email

* Add feature flag that pegs trial length to 7 when disabled

* Add optionality to Identity

* Move feature service injection to identity accounts controller

src/Core/Services/NoopImplementations/NoopMailService.cs

@@ -33,7 +33,8 @@ public class NoopMailService : IMailService
         string email,
         string token,
         ProductTierType productTier,
-        IEnumerable<ProductType> products)
+        IEnumerable<ProductType> products,
+        int trailLength)
     {
         return Task.FromResult(0);
     }