Add RSA support for smartcard-backed SSH authentication (fixes: Issue #4)

[norbusan]

Previously the OpenPGP card SSH auth path handled only Ed25519 and ECDSA keys and rejected RSA. Adding RSA required two changes:

• Algorithm negotiation. sshj advertises RSA keys under a negotiated algorithm name (rsa-sha2-512, rsa-sha2-256 or ssh-rsa) that differs from the key's base type, and the name in the signature blob must match the one in the request. The old override signed with KeyType.toString(), which is always ssh-rsa (SHA-1) and mismatches what putPubKey advertised. Because the card replaces both putPubKey and putSig, CardSshAuthPublickey now tracks its own copy of sshj's KeyAlgorithm queue (peeked for the current algorithm, dropped in shouldRetry) so the advertised algorithm, the signed algorithm and the retry fallback stay in lockstep.

• Card signing input. For RSA the signer builds a PKCS#1 v1.5 DigestInfo (DER hash prefix plus digest, hash chosen from the negotiated algorithm) and hands it to INTERNAL AUTHENTICATE; the card applies the PKCS#1 padding and modular exponentiation and returns the raw modulus-sized signature, which is used directly as the SSH rsa_signature_blob.

[norbusan]

I can confirm that with this change, I was:

• able to import an RSA key from a Yubikey
• able to use the RSA key to clone a remote pass repo
• able to use the RSA key to decrypt a secret

[norbusan]

I have now restructured the code, added a few commits regarding security rework, and created a PR agrahn#977
The original code changes are still attributed to you.

[valasiadis]

I'm very sorry this went past me. I'm still going to review this in a couple of hours when I get home and merge it. That way, there's still a downloadable app version with RSA support until your PR is merged upstream

[valasiadis]

@norbusan I've reviewed the changes and merged them. If you want to, I'd like to merge your other changes too, to keep this repo in a consistent state until your upstream PR is merged.