Add EVP_PKEY_supports_digest_nid()

Rather than relying only on mandatory default digests, add a way for
the EVP_PKEY to individually report whether each digest algorithm is
supported.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7408)