feat: make scram an optional feature

[sunng87]

This patch makes scram an optional feature completely as it introduces a few cypto libraries you might not want if you do not use scram in your application.

cc @serprex , this changes some behaviour introduced in #179

[serprex]

scram should probably be enabled by default, tokio-rustls needs to bring in either ring or aws-lc-rs anyways, so even if someone disables scram they'll end up needing to specify rustls dependency themselves to enable a crypto provider, so even if someone disables scram you'll want separate ring/aws-lc-rs features

[sunng87]

Thanks for the notes. I will be thinking about that.

[sunng87]

The updated feature sets:

• server-api-aws-lc-rs as default, to align with upstream's defaults. It includes tokio-rustls with aws-lc-rs turned on
• server-api-ring as alternative to use ring for rustls
• scram as optional feature to enable scram authentication, it has to be used together with either server-api-aws-lc-rs or server-api-ring

[serprex]

x509-certificate brings in ring still so scram will always bring ring & aws-lc-rs unless using server-api-ring. But that's implementation detail, maybe in future x509-certificate can add aws-lc-rs feature. Otherwise LGTM