Add the ability to add a custom extension on an SSL object

Add the ability to add a custom extension on an SSL object

Previously we could only do this at the SSL_CTX level. We add the ability
to also do this on an SSL - but only for internal code.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19748)