Messages: Add new message fields to message spec

[splindsay-92]

Updated Message class in the textile/features.textile file to support new message fields related to materialisation.

Additions to the Message class:

• Added the action field to represent the type of the message, using the MessageAction enum. (
• Introduced the serial field to store the serial number of the message.
• Added refSerial and refType fields for messages that reference other messages.
• Included updatedAt and updateSerial fields to track the update and deletion ordering.
• Added the operation field, which is a object containing optional fields such as clientId, description, and metadata.