Make serde_yaml optional or configurable

[bjchambers]

What is the problem your feature solves, or the need it fulfills?

serde-yaml has been deprecated (https://github.com/dtolnay/serde-yaml). Many projects are moving to maintained alternatives -- serde_norway] or serde_yaml_ng.

It would be nice if the YAML dependencies in pingora were either optional (behind a feature flag) or configurable. This would allow projects that have upgraded to remove the unmaintained depedency from their transitive dependencies to either omit YAML support or choose to use the same package they have moved to.

Describe the solution you'd like

Make serde_yaml dependency optional (with a yaml feature) and/or add optional dependencies for other libraries.

[johnhurt]

Making the configuration crate optional is a little difficult since it's so embedded in the core of the framework, but we will make it a point to get rid of the deprecated crate in favor of the more current one.