Disable registration on synapse but still be able to invite guest to element call (url and waiting room)

### Your use case

#### What would you like to do?
I would like to disable registration on my synapse server but still be able to invite guests on a call I launch on Element call.
#### Why would you like to do it?
Because my synapse server is small, only for the need of my little company (4 people) so I don't want to open registration but we would need sometime to invite people to a meeting on a call giving them a link.
#### How would you like to achieve it?
By adding these kind of config line to homeserver.yaml :
```
enable_registration: false
allow_guest_access_to_call: true
url_element_call_server: call.matrix.server-name.xyz
turn_allow_guests: true
```
And be able to config in element-call that a user who started a call can invite guests (and accept them when they are on a kind of waiting room).

### Have you considered any alternatives?

I created a second synapse server with registration open dedicated to our element-call. That mean I need the double of server resources and I need to configure, set and maintain two synapse server instead of one.
And anybody who knows the url of our element-call could use it and take ressources of our server.

### Additional context

_No response_