Allow subscribing to selected types of event only

**Problem**

All non-blocking events no matter the types are delivered to the endpoint specified by the user.
And it's expected that the user will filter the events by simple if statements.

The rationale behind this design was to simplify the settings so users do not need to think of which events to subscribe and making the feature simpler to use.

Received a feedback from a user that the numbers of events are too much for their server. Putting this as and idea here.

Current List of events:
https://docs.authgear.com/integrate/events-hooks/event-list
- Signup
- Login
- User profile updated
- User disabled/enabled
- User deletion related
- User anonymous status change
- Add/Update/Deletion of identity

While we are not sure if it's good to separate the subscription,
here are some plausible reasons for segregation:
- Access control: event type that might be sensitive and should only be viewed by one end point
- Event types are in different order of magnitude in terms of loading

**Appetite**
6 weeks

**Solution**

(Still in idea phase)

To allow the user to select and subscribe to certain events based on types

**Rabbit holes**
// Details about the solution worth calling out to avoid problems

**No-goes**
// Functionality or use cases we intentionally aren’t covering to fit the appetite

Start with the current event list at docs.authgear.com/integrate/events-hooks/event-list and trace how all non-blocking events reach the user-specified endpoint. Define the supported event-selection behavior and verify that an endpoint receives only the selected types while the existing event categories remain covered.

Written by the indexing model from the issue text.