Allow modules to listen to alias creation

This issue has been migrated from [#13358](https://github.com/matrix-org/synapse/issues/13358).

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**Description:**

On the Matrix instance I'm managing, we tightly restricted who can create aliases: only a handful of people are allowed to create new ones, so we don't end up with people abusing it.

We would like to relax those rules a little, give more room for people to create new aliases with our domain name, but we would like to monitor the aliases creation so we can have a look at what is happening if we see something fishy.

A synapse module could help us by notifying us in a "control room" whenever a new `#{room_name}:{our_domain.tld}` alias is created. At the moment, the modules API doesn't support listening for new aliases creation. Adding such an API would allow us to create said module.