`arti_client` does not compile without default features

Summary

When disabling default features, arti_client does not compile because it references PreferredRuntime without cfg.

Steps to reproduce:

1. Add arti_client with disabled default features to project
2. Try to compile

What is the current bug behavior?

Compilation fails.

What is the expected behavior?

Compilation succeeds.

Environment

• Version: 0.7.0
• Operating system: Manjaro Linux
• Install method: cargo dependency
• etc...

Relevant logs and/or screenshots:

Possible fixes:

• Feature gate use of the PreferredRuntime type
• Provide a dummy implementation of PreferredRuntime when no features are activated