Dehydrated devices v2 - phase 2 (Element Web R + Legacy iOS only)

[uhoreg]

After initial dehydrated devices v2 lands, we will need still to:

• change dehydrated device format to be less libolm/vodozemac-specific
• update Client-Server API
  • use GET instead of POST for getting events
  • signal end of pagination by omitting token
• cache dehydration key, so that when you reload, you can replace dehydrated devices when you reload
  • caching to be done in Rust crypto crate
  • js-sdk will need to detect if Rust has key cached, and start dehydration process if so
• share key with your other devices after verifying
• look into not encrypting to dehydrated device if receiving user has other active devices
• add UI for enabling/disabling device dehydration (not just use .well-known)
• ensure that things don't break if resetting SSSS has an error (Add support for device dehydration v2 matrix-org/matrix-react-sdk#12316 (comment))

At some point, we also need to rip out the v1 dehydration