Implement subscriptions

[neekolas]

TL;DR

Implemented association change notifications for MLS groups through NATS messaging system.

What changed?

• Added support for association change notifications in the identity service
• Implemented NATS subscription handling for association changes
• Added new topic validation and handling for association changed messages
• Created new topic utilities for association change management
• Modified subscription validation to exclude association change topics from general subscriptions

How to test?

1. Start the XMTP node with MLS enabled
2. Subscribe to association changes using the identity service
3. Trigger an association change in an MLS group
4. Verify that association change notifications are received through the subscription

Why make this change?

This change enables real-time notifications when group associations change in the MLS system, allowing clients to stay synchronized with group membership changes and maintain accurate group state information.

Issues

#410

[neekolas]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add example usage of the SubscribeAssociationChanges API #415
• Start publishing AssociationChange events #413
• Implement subscriptions #412 👈 (View in Graphite)
• Stub out new AssociationChange API #411
• main

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