provider: add OpenSSL::Provider#add_conf_parameter

[rhenium]

Expose OSSL_PROVIDER_add_conf_parameter() for OpenSSL 3.5. This allows setting parameters for an OpenSSL provider directly from Ruby code.

Currently, configuring providers requires a configuration file and setting the OPENSSL_CONF environment variable before OpenSSL is initialized.

Closes https://github.com/ruby/openssl/issues/874