Add notification management

[realvarx]

Is your feature request related to a problem? Please describe.

One crucial aspect in the development of the SDK is the integration of the notification functionality, encompassing the ability to send, receive, and monitor notifications. Prior to progressing with the porting of tools that rely on this feature, such as sshnp, it is highly recommended to prioritize the development of this functionality.

Describe the solution you'd like

Create Notification and NotificationService classes.

Original Dart/Java SDKs code references:

• AtNotification - Dart
• NotificationServiceImpl - Dart
• Metadata - Dart
• Constants - Dart
• Metadata - Java
• AtNotification - Java
• feat: NotificationService at_java#63
• AtMonitorConnection - Java

Task list (not necessarily complete or in the correct order):

(Steps inspired by @JeremyTubongbanua 's atsign-foundation/at_java#63 changelog)

• Create AtNotification class
• Expand Metadata class (new parameters)
• Create NotifyVerbBuilder
• Create NotificationService class, and get_notification_service method for AtClient
• NotificationEnums

[realvarx]

Just to keep the issue updated, notification management system was implemented in #21 and @Xlin123 added diverse features such as notify verb, more metadata parameters, iv and tests in #31.