fix digest mapping for scram auth

[fffonion]

From Kong#20.

In OpenSSL 3.0 where RSA-SHA256 is no longer a valid digest name. This commit fix such use case.
This fix also covers the cases like ecdsa-with-X so the previous block so the manual mapping could be removed.

But I have two thoughts needs discussion:

• This change adds the dependency of lua-resty-openssl even if self.sock_type is not nginx and not cqueues, is that okay?
• I might need some help on adding test of this in CI