Let Element Call widget set session memberships

[AndrewFerr]

Grant Element Call widgets permission to set device-specific session membership state events.

Signed-off-by: Andrew Ferrazzutti [email protected]

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [x] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).

[AndrewFerr]

Related change: https://github.com/element-hq/element-call/pull/2459 This lets Element Call widgets actually request this capability.