Allow starting EC calls with new room header

[toger5]

Your use case

With the new room header we need a way to start new issues.
With the ongoing effort to move to EC by default.
@fkwp and I decided we would like to base this on two conditions:

• If the EC group call feature flag is turned off, we never use EC
• In the EW settings json one can configure what room type uses which call system (jitsy, EC, Legacy) Clicking call uses the configured option.

This keeps the UX the same (simple, no popup and also the behaviour is the same) but it gives the option to move different deployments to different call systems.

[toger5]

https://github.com/matrix-org/matrix-react-sdk/blob/7efd2476569acaa655375278670e7fa1dc7ef5bb/src/utils/room/placeCall.ts#L49-L53

It seems like a dropdown could be an option as well once its part of Compound.

[toger5]

This is now implemented