Element-R: Support staged rollout of migration to Rust Crypto

[BillCarsonFr]

Some sort of mechanism (config.json setting, probably) to migrate a given percentage of users to rust crypto.

This could be set to 100% to migrate all users to Rust.

[richvdh]

Basic ideas on this:

• Add a new setting to matrix-react-sdk/src/settings/Settings.tsx called migrateToRustCryptoRatio or something, which takes a value from 0-100. (See also https://github.com/matrix-org/matrix-react-sdk/blob/develop/docs/settings.md which documents how the settings system works.)
• In initClientCrypto, if feature_rust_crypto is not already enabled:
  • Hash the device ID to produce an integer (say, 32 bits). The precise details of the hash algorithm are unimportant, but the result needs to be reasonably evenly distributed over the output range.
  • If the hash is greater than (2**32 / 100) * migrateToRustCryptoRatio, we will migrate this session. Set useRustCrypto = true, persist it to the settings store, and carry on with the init code.