Room: Add set_name and set_topic methods

name and topic are exposed as read-only properties but can't be updated. Bots often need to write room metadata dynamically (e.g. live status in the topic).

### Proposed API
```python
async def set_name(self, name: str) -> None
async def set_topic(self, topic: str) -> None
```

### Example
```python
await room.set_name("General")
await room.set_topic("Last updated: 13:00 UTC")
```

### Before opening a PR
- Write unit tests
- Run `mypy matrix/`
- Run `pytest tests/`
- Run `black .`