feat: create an EC::PKey from an EC::Point

[mcr]

It is implied that OpenSSL::PKey::EC.new can take a public key/EC::Point to form a key, but I could not make it work. Since 3.x wants keys immutable, they have to be initialized at creation, and this allows for an EC::Point to be provided. RSA keys do not have the same division between PKey and Point.