Providers: for the digest_final operation, pass a output buffer size

This allows the provider digest_final operation to check that it
doesn't over-run the output buffer.

The EVP_DigestFinal_ex function doesn't take that same parameter, so
it will have to assume that the user provided a properly sized buffer,
but this leaves better room for future enhancements of the public API.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8747)