Add new event subscription methods with sync/async/noread options

• Polish commits
• Function level documentation
• Add tests for new interfaces
• #1402 (closed)

Todos from our meeting on 2025-05-23, see https://mattermost.hzdr.de/tango-controls/pl/jks13tsysiruzjqqru3ho8qtdr:

• Only use one thread for the async case, unify with the subscribe case from an event callback. Maybe this needs more fine grained locking of the event_not_connected map?!
• Add stateless to EventSubMode
• Test cases for all event sub modes where the event is not enabled on the attribute
• Tests: More test assertions for the returned event data
• Tests: rename k_XXX constants
• Include !1460 (merged) here
• Async/AsyncRead: Call callback when the subscription is finished. Introduce new member for that. An int (?!) could count the number of calls to the callback.
• Followup issue for batched event subscriptions, #1447
• Add entries for JSON of QueryEventPerformance if required
• cleanup commits
• More documentation on new event modes

Other todos:

• Use create_zmq_event_consumer unconditionally in subscribe_event methods
• Rebase once !1462 (merged) is merged
• Don't add match overloads accepting a pointer as these don't output stringified objects anymore, adapt tests

Done:

• Respond to code review from TI, flesh easy parts out into !1452 (merged)

Close #1344 (closed)