Add config option to disable registration/guest users (and login)

### Your use case

We currently disable registration and guest users via synapse `homeserver.yaml` with

```
enable_registration: false
allow_guest_access: false
```

and also at Element Web `config.json` via

```
[...]
"disable_guests": true,
"disable_login_language_selector": true,
"disable_3pid_login": true,
[...]
"sso_redirect_options": {
"immediate": true,
"on_welcome_page": true
},
[...]
```

Element Call landing page offers registration, guest login and also offers a login via user+password despite we only support SSO.

#### What would you like to do?
Disable certain options.

#### Why would you like to do it?
Because we don't allow this and also set the options on the various other parts like synapse and Element Web.

#### How would you like to achieve it?
Have additional config options in Element Call `config.json` to disable registration and guest login and either also regular login and/or redirect to SSO like Element Web can be configured to do.

### Have you considered any alternatives?

No.

### Additional context

_No response_