Send call notify event with extensible event fallback

[toger5]

Your use case

If I start a call, clients that do not support matrixRTC calls should still get informed and be able to interact with the sender
about how they can join the call.
As described in: matrix-org/matrix-spec-proposals#4075

Relevant code location:
https://github.com/matrix-org/matrix-react-sdk/blob/9683693607fac00b7f5a8b29479486452a57d92f/src/models/Call.ts#L865-L877

Have you considered any alternatives?

No response

Additional context

No response

[robintown]

We should do this!

[robintown]

@Half-Shot Just checking in, did you find any time for this? (assuming not, we've moved it to our backlog)