invite guest to a call

### Your use case

#### What would you like to do?
Create an Element Call video room, get a link that we can share to invite guest

#### Why would you like to do it?
Invite guest to a video conf without need of an other tool than synapse + Element

#### How would you like to achieve it?

### Have you considered any alternatives?

Install an other synapse server + Element Call (but registration need to be open, and it makes the need of 2 synapse server) or a jitsi or a bbb server...

### Additional context

_No response_

The issue names no files, tests, or entry points, so first clarify how guest access should work and which existing Element Call flow should change. Define done as creating a video room whose shareable link lets guests join without another Synapse server, open registration, or a separate calling service.

Written by the indexing model from the issue text.