Refactor verification request code

[bwindels]

Part of element-hq/element-web#11224

This extracts most of the verification request code from crypto/index and puts it into it's own VerificationRequest class with the specificities of whether to use to_device verification or verification over DM into a Channel class. Care has been taken to maintain the same public API from the crypto module and the request object passed as an argument of the crypto.verification.request event.

The next step will be add support for the m.key.verification.ready event, and to decorate verification events with a verificationRequest property and use this to drive toasts and verification timeline tiles in the react-sdk, removing the KeyVerificationStateObserver.

[bwindels]

(cleared myself as assignee to avoid misunderstandings that this is somehow waiting for me to do anything; it's not, and fully ready for review)

[dbkr]

Massive thanks for moving things out of crypto/index.js! This looks much nicer.