pkey: add support for OpenSSL 3 provider-only pkeys

[rhenium]

This includes two changes:

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pkey: handle EVP_PKEY_KEYMGMT return by EVP_PKEY_id()

For algorithms implemented solely in an OpenSSL 3 provider, without an associated EVP_PKEY_METHOD, EVP_PKEY_id() returns a special value EVP_PKEY_KEYMGMT.

Let OpenSSL::PKey::PKey#oid raise an exception as necessary. Update PKey#inspect to include the string returned by EVP_PKEY_get0_type_name(), if available.

---

pkey: use EVP_PKEY_new_raw_{private,public}_key_ex() if available

Algorithms implemented only in OpenSSL 3 providers may not have a corresponding NID. The *_ex() variants have been added in OpenSSL 3.0 to handle such algorithms, by taking algorithm names as a string.

[junaruga]

Thank you for the PR! I confirmed the OpenSSL::PKey.generate_key works in the following ML-DSA cases!

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("ML-DSA-44")'
#<OpenSSL::PKey::PKey:0x00007fc4bc283ba0 type_name=ML-DSA-44 provider=default>

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("mldsa44")'
#<OpenSSL::PKey::PKey:0x00007f8a4c6f3b40 type_name=ML-DSA-44 provider=default>

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("ML-DSA-65")'
#<OpenSSL::PKey::PKey:0x00007f8456483b90 type_name=ML-DSA-65 provider=default>

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("mldsa65")'
#<OpenSSL::PKey::PKey:0x00007fda44e83b38 type_name=ML-DSA-65 provider=default>

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("ML-DSA-87")'
#<OpenSSL::PKey::PKey:0x00007f8685af3be8 type_name=ML-DSA-87 provider=default>

$ ruby -I ./lib -e 'require "openssl"; p OpenSSL::PKey.generate_key("mldsa87")'
#<OpenSSL::PKey::PKey:0x00007f20df6e3bb8 type_name=ML-DSA-87 provider=default>