Fix fastmail username forwarder by changing url: to forDomain: in the API request (#6827)

Url is incorrect and should not be used per the fastmail integration.

> If your product supports deep links, the url property should be a deep link to the 
> credential or other record related to this masked email address in your product. If 
> deep links are not supported, leave this null. Do not set this to the domain of the 
> site the user is using the masked email for, see above and use forDomain for this.

At present, the username generator stores the credential locally, so there's no way 
to deeplink to the credential reliably. We should look into supporting url if, in the 
future, we sync the key across clients.

libs/common/src/tools/generator/username/email-forwarders/fastmail-forwarder.ts

@@ -35,7 +35,7 @@ export class FastmailForwarder implements Forwarder {
               "new-masked-email": {
                 state: "enabled",
                 description: "",
-                url: options.website,
+                forDomain: options.website,
                 emailPrefix: options.fastmail.prefix,
               },
             },