Redirect EVP_DigestInit to EVP_DigestSignInit_ex if appropriate

Redirect EVP_DigestInit to EVP_DigestSignInit_ex if appropriate

Prior to OpenSSL 3.0 calling EVP_DigestInit_ex() on an mdctx previously
initialised with EVP_DigestSignInit() would retain information about the
key, and re-initialise for another sign operation. To emulate that we
redirect calls to EVP_DigestInit() to EVP_DigestSignInit_ex() if
appropriate.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12850)