Add function to parse RSA public key in PKCS1 format

[emtammaru]

There's several different ways to do this in different versions of openssl. In openssl v3, it can be done with PEM_read_RSA_PUBKEY and EVP_PKEY_get_bn_param. To support v1, I had to go digging a little further. There's an alternate function d2i_RSAPublicKey_bio, but it doesn't work for our specific format. The function d2i_RSA_PUBKEY_bio is specifically for parsing keys in PKC#1 container which is what we need.

I generated the testdata for the unit test with the following commands:

openssl genrsa -out rsa.private 1024
openssl rsa -in rsa.private -out rsa.public -pubout -outform DER