Adding TLS group name retrieval

Adding TLS group name retrieval

Function SSL_group_to_name() added, together with documentation and tests.
This now permits displaying names of internal and external
provider-implemented groups.

Partial fix of #13767

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13785)