Space: Add a `broadcast` method

[PenguinBoi12]

A broadcast method could be a good way to let bot authors send announcements or notices to every room in a space without looping manually.

Proposed API

async def broadcast(
    self,
    content: str,
    *,
    raw: bool = False,
    notice: bool = False,
) -> list[Message]:

Returns a list of Message objects (one per room the message was successfully sent to), consistent with how Room.send works.

Example

space = bot.get_space("!space123:matrix.org")
messages = await space.broadcast("Maintenance starts in 10 minutes.", notice=True)

Before opening a PR

• Write unit tests
• Run mypy matrix/
• Run pytest tests/
• Run black .

[grivera64]

Can I be assigned to this issue? Currently working on a PR

[grivera64]

Thanks! I made a PR at #86