Streamline `message_id` and `in_reply_to` fields of `Patch` struct

[davidbtadokoro]

Context:

The message_id and in_reply_to fields of Patch are of type MessageID and Option<MessageID>, respectively.

Proposal:

Maybe we can streamline the implementation by flatting these fields as String and Option<String>. We need to consider if there will be scenarios in which having a dedicated type for MessageID will be necessary.

Setup:

• Project branch: unstable
• Project commit hash: 0fb93db
• rustc version: 1.79.0
• cargo version: 1.79.0

[ivinjabraham]

These fields will be used in the new review feature and we will probably need the structs then.