@gkc

I have a blocker with the last 2 outstanding tasks to merge this PR. They are related to validating the NotificationParams object. In my opinion, I think it is okay to ignore for now because the only work that is left is validating the NotificationParams object. Notifications still work in this PR. I've left comments and the ground work is laid out in case we really want to add Notification validation in the future. If you think this PR is okay, it is good to merge.

More Info on the Issue:

Unable to implement the use of static method AtClientValidation.validateNotificationRequest(NotificationParams, String, int) in line 49 of NotificationServiceImpl.java

The summary of the problem is that the method requires the RootURL of the atDirectory server but the AtClient instance does not have access to this variable because the core factory method (that actually instantiates the AtClientImpl instance) does not store it because it takes the remote secondary address as an argument (as opposed to the root url).

Sources:

1. Dart AtClientValidation using AtClientPreference to get rootDomain and rootPort https://github.com/atsign-foundation/at_client_sdk/blob/f8998895e11a4a45c8869a2c7433b5a140a85c4b/packages/at_client/lib/src/util/at_client_validation.dart#L133
2. Dart NotificationServiceImpl actually using the method https://github.com/atsign-foundation/at_client_sdk/blob/f8998895e11a4a45c8869a2c7433b5a140a85c4b/packages/at_client/lib/src/service/notification_service_impl.dart#L339
3. Java AtClient factory method that actually creates the instance

   at_java/at_client/src/main/java/org/atsign/client/api/AtClient.java

   Line 106 in d280422

   static AtClient withRemoteSecondary(AtSign atSign, Secondary.Address remoteSecondaryAddress, boolean verbose) throws AtException {

   (note that this method does not ask for a rootUrl, there are other factory methods that ask for a rootUrl as an argument but eventually call this method).

Branch has merge conflicts that need to be resolved