support for U2F signing of raw data; I will make the PR with a little guidance

[palmin]

To support ssh public key authentication with [email protected] keys in my SSH client there needs to be a way to perform U2F signing on raw data instead of JSON.

I have implemented changes to yubikit-ios to support this in 0694403 but I imagine you have some wishes on how raw U2F signing should be added to the SDK.

Please let me know how you would like such a change to be implemented or if the current commit is acceptable. I want this to be as easy for you as possible.