feat: Add full feature

[tottoto]

Motivation

Makes it easy to turn on all features, like tokio and hyper.

Solution

Adds full feature, which enables all features.

[jplatte]

I'm not a big fan to be honest. What motivated this PR?

[tottoto]

This allows users to easily enable all features and start development.
As an additional benefit, I think it makes it easier in CI to test jobs that should not enable private features, instead of relying on --all-features.

[jplatte]

I understand the general idea behind this Cargo feature, but what made you open this PR? Was there a project where you actually wanted to use this yourself? Did you start writing some tutorial or similar material where you wanted to simplify things? Something else?

[tottoto]

When testing, I sometimes create new environments and try out several features. In such cases, I think it would be convenient to have a way to enable multiple features at once, instead of looking up the required features one by one and enabling them individually, or maintaining a full list of all features and copying it each time.