Ref #31: Initial support for Hybrid transport (iOS only, non persistent)

[AlfioEmanueleFresta]

An initial rough implementation of hybrid transport - formerly known as Cloud Assisted Bluetooth Low Energy, or caBLE.

Includes

• Generates QR code for QR-initiated transactions
• Adds WebAuthn example which prints QR code to terminal (examples/webauthn_cable.rs)
• Adds submodule fork of snow (noise protocol framework) with NIST P-256 curve implementation, as required by CTAP spec (see Add support for NIST P-256 curve mcginty/snow#185)

Limitations and follow ups

• Unable to discover Android devices (created issue Hybrid transport: Can't discover Android device #52)
• Does not yet support state-assisted transactions (part of FIDO2 over Hybrid Transport (aka Cloud-Assisted BLE, or caBLE) #31)
• Clean up (part of FIDO2 over Hybrid Transport (aka Cloud-Assisted BLE, or caBLE) #31)

[AlfioEmanueleFresta]

@msirringhaus would love your thoughts if you have the time for a review. Will try and merge this by tomorrow EOD otherwise.

[msirringhaus]

Some minor things. I didn't do a full review if everything is according to the spec, though. I'm guessing, if it works, it can't be that wrong :)

[AlfioEmanueleFresta]

Thanks for your feedback @msirringhaus! Addressed your comments - I'll rebase and land tomorrow.