Add the X509v3_cache_extensions() function

Add the X509v3_cache_extensions() function

Various functions cause the results of processing extensions to be
cached. The processing itself requires a libctx, and so this implicit
caching means that the default ctx is used which can lead to failures.
By explicitly caching the extensions we can specify the libctx to be used.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11409)