js-sdk voip architecture: Input validation & types

### Your use case

https://github.com/matrix-org/matrix-js-sdk/blob/fc2671d8538a3339ca5925ef42e3ba6102f7e8f2/src/matrixrtc/CallMembership.ts#L40 takes a typed data structure and does further validation on it that should be unnecessary because of the types. This is a bit weird and confusing, but is because we are casting the raw content from the event into a CallMembershipData and then having the constructor take a typed interface, so we are technically lying about the type of the data between those points.

Now that I think about this more, I suspect the neatest solution would be to have a separate validator function that takes `unknown`, validates it and returns `CallMembershipData` or throws, then you can construct the CallMembership with a validated CallMembershipData.

Either way, we should do something less confusing, but still actually do explicit validation which we have often neglected in the past.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_