Add CallNotifyEvent to support matrixRTC ringing

[toger5]

This adds the CallNotify event type and the corresponding ICallNotifyContent

Also this adds a sessionId to matrixRTCSessions. The sessionId is the value of call_id of the first membership. The memberships for a session are filtered by that id. So we can take either membership to calculate the sessionId

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Add CallNotifyEvent to support matrixRTC ringing (#3878). Contributed by @toger5.

[dbkr]

The bulk of this change seems to be adding the session ID I think. I see the desire to export a common type for the notify events in js-sdk but it seems a bit weird for it to just export the type and then never actually use it?

[dbkr]

I would make this clear that this is only for use within the client and won't necessarily be the same between different clients or the same client restarted. Also probably want docs on the getter as that's where the public-facing API is.

and probably, "the oldest member event at the time the MatrixRTCSession object was created", although having read further I guess this isn't really correct either. "The first membership the MatrixRTCSession sees?"

[dbkr]

Actually reading the react-sdk PR for context, isn't this just callId? ie. the discriminator used to identify different user calls in the same room? I would just call it callId rather than making up new terminology for it? The callIds for memberships in a session must be the same by definition, or they would belong in a different session.

[toger5]

yes exactly this is just the call_id from the matrix rtc session.
Since the MatrixRTCSession object is very generic I felt like having call specifics in here is odd.