include/ipc: Add function to list subscriptions of an endpoint

include/ipc: Add function to list subscriptions of an endpoint

The IPC module does not provide a method to get the topics that an
endpoint is subscribed to, requiring the user to track the topics
themselves, if they need this information.

This commit adds the `ipc_endpoint_get_subscriptions()' method,
which returns the list of topics that an endpoint is subscribed to.