Define aliases private_key? and public_key? for OpenSSL::PKey::EC

I've discovered this bug [when I was trying to work with GOST keys](https://bugs.ruby-lang.org/issues/9830) (which are EC keys), and I was unable to do signing as `sign` method in `OpenSSL::PKey` class (superclass of `OpenSSL::PKey::EC`) calls `private_key?` method on self, but `OpenSSL::PKey::EC` doesn't implement it.