Generic attributes in change tracking

[shashankbrgowda]

Currently Apollo Havana Integration script heavily relies on change tracking to process the annotation changes. It'll be really helpful If we have a generic attribute object for users/consumers to store some state for each change.

For example, after processing a location start change for a feature we need to mark it as complete. For this It'll easier to maintain the state in the changes collection itself instead of maintaining the state in a separate system.

{
...,
  "attributes": {
    "havana_processed": true,
    "some_other_key": "value",
  },
}

[shashankbrgowda]

This would also help us to keep track of failed changes. Then we can investigate further on why that change has failed, we can't do this if we query changes using change sequence number.