support registering yubico otp keys

support registering yubico otp keys

In PBS we don't support this, so the current TFA API in rust
does not support this either (although the config does know
about its *existence*).

For now, yubico authentication will be done in perl. Adding
it to rust the rust TFA crate would not make much sense
anyway as we'd likely not want to use the same http client
crate in pve and pbs anyway (since pve is all blocking code
and pbs is async...)

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>