Support untruncated responses

[stv0g]

I would like to generate a SHA256 HMAC using the app and retrieve the full digest.

This seems to be currently not possible.

YKOATH supports this by enabling/disabling the dynamic truncation via P2 of the calculate instruction. However, The Trussed secrets apps unconditionally performs dynamic truncation.

This would also allow us to ditch the Yubico-specific HMAC instruction and just use TOTP credentials where we pass a custom challenge.

The Yubico-specific HMAC instruction currently only supports SHA1 which is a no-go for my use case :/

[stv0g]

Btw, my use case is the calculation of AWS HMAC request signatures:
https://docs.aws.amazon.com/AmazonSimpleDB/latest/DeveloperGuide/HMACAuth.html