Added onDismiss callback

[Do-Repo]

onDismiss callback was needed inorder to trigger an event when notification is dismissed.
This comes in useful if you want to add sound with the notification as in my case. I needed a callback to know when to stop the sound.

[github-actions]

LCOV of commit a43c4ca during Testing #39

Summary coverage rate:
  lines......: 68.7% (134 of 195 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                            |Lines       |Functions  |Branches    
  Filename                                  |Rate     Num|Rate    Num|Rate     Num
  ================================================================================
  lib/src/in_app_notification.dart          |74.4%    125|    -     0|    -      0