Add APIs for custom X509_LOOKUP_METHOD creation

OpenSSL 1.1.0 made the X509_LOOKUP_METHOD structure opaque, so
applications that were previously able to define a custom lookup method
are not able to be ported.

This commit adds getters and setters for each of the current fields of
X509_LOOKUP_METHOD, along with getters and setters on several associated
opaque types (such as X509_LOOKUP and X509_OBJECT).

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6152)