rsa: verify PUBKEY_ENC_UNKNOWN via the raw compare path (fixes gpg/apt RSA verify)

[MarkAtwood]

Fixes #28.

wc_rsa_verify() rejected every RSA signature whose libgcrypt encoding context
is PUBKEY_ENC_UNKNOWN, because the switch (ctx.encoding) only handled the
named encodings (PSS / PKCS1 / PKCS1_RAW / RAW / OAEP) and routed everything else
to default: rc = GPG_ERR_BAD_SIGNATURE.

GnuPG's OpenPGP RSA verification supplies the full pre-encoded PKCS#1 v1.5
message and leaves ctx.encoding == PUBKEY_ENC_UNKNOWN, so all GnuPG/apt RSA
signature verification failed (keys "Unusable", "1 bad signature"). Stock
libgcrypt performs the raw mpi_cmp(result, data) for all non-PSS encodings;
this PR routes PUBKEY_ENC_UNKNOWN into the same wc_RsaDirect + mpi_cmp
group.

Test

export GNUPGHOME=$(mktemp -d)
gpg --batch --pinentry-mode=loopback --passphrase "" \
    --quick-generate-key "test@example.com" rsa2048 sign 0
gpg --check-sigs
# before: "gpg: 1 bad signature"
# after:  clean

Notes

• The crypto itself is correct: an OpenSSL-produced PKCS#1 v1.5 RSA-SHA256
  signature verifies through native wolfCrypt with the key loaded via
  wc_RsaPublicKeyDecodeRaw + wc_RsaDirect(RSA_PUBLIC_DECRYPT) (recovered EM is
  byte-for-byte correct). Only the shim's encoding dispatch was wrong.
• A more defensive alternative is to make the default: case perform the raw
  compare (matching stock libgcrypt) instead of adding an explicit case — happy
  to switch to that form if preferred.

[Copilot]

Pull request overview

This PR fixes RSA signature verification failures in the wolfCrypt-backed wc_rsa_verify() path when the libgcrypt encoding context is PUBKEY_ENC_UNKNOWN, which is commonly encountered during GnuPG OpenPGP RSA verification (and thus impacts apt signature verification via gpg/libgcrypt).

Changes:

• Route PUBKEY_ENC_UNKNOWN through the same “raw public operation + mpi_cmp” verification path used for other non-PSS encodings, instead of treating it as a bad signature.

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