bindings/crypto-nodejs: add functions for encrypting custom to-device messages for receivers

[turt2live]

This will be needed for things like MSC3401 VoIP where arbitrary event shapes will need encrypting and sending as to-device messages.

[turt2live]

not a blocker for the bot-sdk bindings, but also higher priority for the medium term.

[Hywan]

Hey :-),

I don't understand the feature request. Do you know if the matrix-sdk-crypto crate supports that already?

[turt2live]

I don't know if it supports it, but we do need a way to encrypt arbitrary toDevice messages :D