pgmoon.crypto better support for resty.openssl / resty.string libraries

[bungle]

Summary

• Adds random_bytes function to pgmoon.crypto compatibility layer and makes use of it in pgmoon.init
• Adds x509_digest function to pgmoon.crypto compatibility layer and makes use of it in pgmoon.init
• Adds resty.openssl support to hmac_sha256
• Adds resty.openssl and resty.string support to digest_sha256

We don't anymore ship luaossl with Kong and this causes our users issues with scram-sha-256 authentication as reported in: Kong/kong#8259

This PR fixes it so that it works with resty.openssl library too (the one that is shipped with Kong).

[bungle]

Hi @leafo, if you have time, could you give some eye to this? I can make changes as requested.

[leafo]

Thanks, looks good

[leafo]

Sorry about the delay publishing, 1.14 is now out on luarocks and opm