feature request: access to "obj_create" and raw OIDs when given 'UNDEF'

[trevormarvin]

using (py)OpenSSL version 19.1.0 on my side

1. Within the OpenSSL.crypto.X509Extension class, a method like .get_oid() to return the raw OID for an extension that is returning the short name UNDEF would be nice. I have not found another way to get this data from these objects when sifting through them (when using the OpenSSL.crypto.X509.get_extension_count() method and then indexing through each one). I see past requests here from about 2014 (#138) with similar requests; and in respect of the concern to not break reverse compatibility by of returning the OID when the name is not available, I suggest a new method to get the raw OID.

2. Access to the OBJ_create method of the underlying library. I hope I'm reading this correctly; (I'm strong in Python, not C) it appears that there's a method in the underlying library to temporarily insert OID definitions? I have not found this method in the current pyOpenSSL library. This could mitigate the first issue I have mentioned above, at least for my purposes.