Support CKA_PUBLIC_KEY_INFO

[dengert]

PKCS11 C_GetAttributeValue for CKA_PUBLIC_KEY_INFO returns with zero length.
This comment appears 3 times in the code: "// TODO: CKA_PUBLIC_KEY_INFO is accepted, but we do not calculate it."
one is here.

The comment: "// i2d_PUBKEY incorrectly does not const the key argument?!" appears here.

https://docs.openssl.org/3.0/man3/X509_PUBKEY_new/ says: "i2d_PUBKEY() encodes an EVP_PKEY structure using SubjectPublicKeyInfo". PKCS11 says: "The value of the CKA_PUBLIC_KEY_INFO attribute is the DER encoded value of SubjectPublicKeyInfo".

https://docs.openssl.org/3.0/man3/X509_PUBKEY_new/#description says: "i2d_PUBKEY() encodes an EVP_PKEY structure using SubjectPublicKeyInfo format."

This should be easy to implement, for a C++ programmer, and it would work on any type of pubkey.

The problems shows up in mods to OpenSC OpenSC/OpenSC#3090 calling the softhsm2 pkcs11 module.