[Task] Disambiguate message_id in IotaDocument

[cycraig]

Description

Add integration_message_id and diff_message_id to IotaDocument.

Motivation

The message_id stored in a resolved IotaDocument is typically the message_id of the last document published to the integration chain. When there are diff chain updates merged in during resolution, the message_id becomes MessageId::Null (all zeroes). This is somewhat correct in that a resolved document with merged diff changes does not have a message_id because it is not technically on the Tangle in that state. Unfortunately, it makes it difficult to apply new updates (to either the integration or diff chain) due to the lack of a message_id to set for previous_message_id.

This proposal is to keep the current behaviour to avoid a breaking change for now and add two new fields: integration_message_id, and diff_message_id. The integration_message_id is equivalent to the current message_id except it is not overwritten when applying diff updates, and diff_message_id is simply the message_id of the last merged DocumentDiff. These ids will need to be set during resolution and care needs to be taken to ensure the Account maintains them too, as it essentially adds more state to the IotaDocument that can become outdated or incorrect.

Edit: this should be part of the larger refactor of document metadata and resolution metadata.

To-do list

Create a task-specific to-do list . Please link PRs that match the To-do list item behind the item after it has been submitted.

• Add integration_message_id and diff_message_id to IotaDocument or metadata
• Add/update tests
• Update examples?

Change checklist

• The feature or fix is implemented in Rust and across all bindings whereas possible.
• The feature or fix has sufficient testing coverage
• All tests and examples build and run locally as expected
• Every piece of code has been document according to the documentation guidelines.
• If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
• If the feature is not currently documented, a documentation task Issue has been opened to address this.