Update rustls

[serprex]

rustls replaced ring with aws-lc-rs as default crypto backend,
expose features to select between the two, along with a feature on whether to enable tls 1.2

[serprex]

as an aside, rust features are supposed to be additive, this is not currently the case with rustls/native-tls features. If both features are enabled the compile fails. Would you be interested in having changes made to rectify that?

I think the fix would be to have each crypto backend use a different Opts (ie, ssl_opts would be ssl_opts_native & ssl_opts_rustls). Could get more complicated with traits, but doesn't seem necessary

[westy92]

Completed as part of #313

[serprex]

@westy92 that PR doesn't include adding ring/aws-lc-rs features now that rustls made their crypto backend configurable. Setting features = [] on rustls will run into problems if application using mysql_async lacks another dependency to same version of rustls setting which backend to include