[PM-4800] Send item domain name to fastmail (#2867)

* Send item domain name to fastmail

- Added a metadata field (forDomain:) to the Fastmail Forwarder API
  request that's set to the domain name of the item being added to the
  vault, or to "" if the username generator is being used in standalone
  mode. This allows the user's Fastmail account to display the domain
  name for the username that was generated.

* Minor changes for readability

* dotnet format

---------

Co-authored-by: ✨ Audrey ✨ <ajensen@bitwarden.com>

src/Core/Services/UsernameGenerationService.cs

@@ -148,6 +148,13 @@ namespace Bit.Core.Services
                                     .GenerateAsync(_apiService, forwardedEmailOptions);
             }
 
+            if (options.ServiceType == ForwardedEmailServiceType.Fastmail)
+            {
+                var fastmailEmailOptions = (FastmailForwarderOptions)options.GetForwarderOptions();
+                return await new FastmailForwarder()
+                                    .GenerateAsync(_apiService, fastmailEmailOptions);
+            }
+
             BaseForwarder<ForwarderOptions> simpleForwarder = null;
 
             switch (options.ServiceType)
@@ -161,9 +168,6 @@ namespace Bit.Core.Services
                 case ForwardedEmailServiceType.DuckDuckGo:
                     simpleForwarder = new DuckDuckGoForwarder();
                     break;
-                case ForwardedEmailServiceType.Fastmail:
-                    simpleForwarder = new FastmailForwarder();
-                    break;
                 default:
                     _logger.Value.Error($"Error UsernameGenerationService: ForwardedEmailServiceType {options.ServiceType} not implemented.");
                     return Constants.DefaultUsernameGenerated;