Add more granular control over real-time updates

[acelaya]

Shlink has supported real-time updates for some time, with support for RabbitMQ, Mercure and Redis pub/sub (ATM of writing this).

Shlink currently supports 4 real-time updates operations:

1. https://shlink.io/new-visit: Get notified every time there's a new visit on any short URL.
2. https://shlink.io/new-visit/{shortCode}: Get notified every time there's a new visit on a particular short URL.
3. https://shlink.io/new-orphan-visit: Get notified every time an orphan visit is created.
4. https://shlink.io/new-short-url: Get notified every time a short URL is created.

1, 3 and 4 are non-controversial "static" channels, but 2 is a dynamic channel pattern which can cause more resources to be needed, specially in contexts with a lot of short URLs.

For example, as reported in #2382, that particular operation can cause too many queues to be created in a RabbitMQ server, which can become resource greedy, specially if the messages are not consumed.

Because of that, it would be useful that users can configure which real-time operations should be enabled, rather than being an all-or-nothing option.