Consider Supporting HMAC in Digest

[micchickenburger]

Consider adding a key field to digest which, when populated, produces an HMAC using the selected hashing algorithm. Presently, an HMAC key can be generated under the encryption tab for use as a digital signature, but keys cannot yet be imported.

HMAC keys are secrets, so it is a little haphazard to enable pasting them into an input field, even if such field is of type password. That being said, importing and using a key to produce a hash as a digital signature is a more cumbersome process.