Invisible crypto: revisit CryptoMode concept

[richvdh]

#4407 introduced a CryptoMode api to the js-sdk, which we think we need to revisit, for a couple of reasons:

1. "CryptoMode" isn't very descriptive (does it refer to the crypto ciphers being used? MLS vs Double Ratchet? or?)
2. More importantly, the exposed options don't actually fit what we need very well.

Instead, we imagine we might have a CryptoApi.SetDeviceIsolationMode, which takes some sort of enum-with-options parameter.

(For Element folks, see brainstorming doc at https://docs.google.com/spreadsheets/d/1OEoqP3PAEsdvPBCYxIxJdp0GZhtLbq75dex-fOVQkO0/edit?usp=sharing)