Message: Add forward method

There's no way to relay a message from one room to another. This is useful for bots that mirror announcements across rooms.

### Proposed API
```python
async def forward(self, room: Room) -> Message
```

### Example
```
target = bot.get_room("!other123:matrix.org")
await ctx.message.forward(target)
```

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