Add example usage of the SubscribeAssociationChanges API

[neekolas]

TL;DR

Added a new example demonstrating how to consume XMTP's SubscribeAssociationChanges API.

What changed?

• Added a new example in Go that shows how to subscribe to and process identity association changes
• Created documentation explaining the identity update stream API's purpose and use cases
• Fixed the identity service implementation to properly handle stream context cancellation

How to test?

1. Run the example locally by connecting to localhost:5556
2. Verify that the stream receives updates when wallet addresses are associated or revoked from inbox IDs
3. Confirm the output shows proper formatting of timestamps and event details
4. Test connection to dev/production networks by updating the connection string and adding TLS credentials

Why make this change?

Applications need a way to monitor identity association changes in real-time to:

• Maintain up-to-date registries of account address to inbox ID mappings
• Build search functionality based on these mappings
• Notify users about new wallet associations with their InboxID
• Support backend services that need to track identity changes across the XMTP network

[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 👈 (View in Graphite)
• Start publishing AssociationChange events #413
• Implement subscriptions #412
• Stub out new AssociationChange API #411
• main

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