Introduce signing guile-ssh functions.

[nicolas-graves]

This is corresponding to the current https://gitlab.com/libssh/libssh-mirror/-/merge_requests/536 (not merged yet, but I expect that to be soon enough).

Here's a usage example :

(use-modules (ssh key)
             (srfi srfi-26)
             (ice-9 textual-ports))

(define (public-keys-equal? pub1 pub2)
  (string=? (public-key->string pub1)
            (public-key->string pub2)))

(define rsakey (private-key-from-file "tests/keys/rsakey"))
(define test-string "Test input\0string")

(call-with-output-file "/tmp/message.sig"
  (cut format <> (sign test-string rsakey)))

(and=>
 (verify
  "Test input\0string"
  ;; "not a signature"
  (call-with-input-file "/tmp/message.sig" get-string-all))
 (cut public-keys-equal? rsakey <>))

[nicolas-graves]

@artyom-poptsov The first commit can already be reviewed/merged, see https://codeberg.org/guix/guix/issues/113 for the reason why it's required.

[nicolas-graves]

@artyom-poptsov It's been merged upstream! Next steps here?