Allow usage of ring instead of aws-lc-rs in tokio-rustls

[Leandros]

Feature Request

Motivation

To allow for not linking to aws-lc-rs (specifically, removing any usages of aws-lc-sys), I'd like to have the ability to chose the used provider from rustls.

Proposal

Tonic pulls in the tokio-rustls dependency, which defaults to use aws-lc as a cryptography primitive provider. However, tokio-rustls also ships with an alternative provider ring. To enable ring, the feature ring needs to be set (and the default features disabled).

To allow this with tonic, tonic would have to expose a new feature that allows choosing ring over aws-lc and forwards the feature flags to tokio-rustls.

Alternatives

Ideally, cargo would allow us to change features of dependencies of dependencies. However, at the current moment this isn't possible.