Hook for role management

## Problem

For backends just like mine which get their user table updated via an Authgear webhook, having an event fired when a user role gets added or deleted would be much nicer, since the webhook can already update the user's info but not their "permissions".

## Appetite

It's pretty simple, just fire a Hook event, just like the other ones.

## Solution

Either one of the two:
- Adding two events: `user.roles.added` and `user.roles.deleted`
- Simply fire the `user.profile.updated` event with the roles in it.

## Success definition

Self-explanatory.

## Rabbit holes

Non-existent.

## No-goes

N/A.

Thanks!!

The issue names no files or tests. Start by tracing the existing Hook events and the user profile update path, then compare how role additions and deletions are represented. Done means role changes reliably produce the agreed Hook event and the webhook receives the updated permissions information.

Written by the indexing model from the issue text.