Add `sign_digest_less_safe` to rsa `KeyPair`

[ldufr]

The goal of this MR is to allow a user to sign a digest without having to have all the message at disposition. I tried to follow the advises given in MR-915, for instance the name of the function.

In addition, I'm not convince the testing code I added is the best or most elegant way to do it, but I didn't want to add custom stuff to support testing. Arguably, it's a little awkward to specify the digest algorithm twice. You might have some recommendation on that.