ServerConf::from_yaml() & Co. should be implemented via a trait

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

Extending `ServerConf` with custom settings works along these lines:

```rust
#[derive(Debug, PartialEq, Eq, Deserialize)]
#[serde(default)]
pub struct MyAppConf {
custom_setting: bool,

#[serde(flatten)]
server: ServerConf,
}
```

The problem is, `ServerConf::from_yaml()` and similar methods are not defined for this structure and have to be reimplemented.

## Describe the solution you'd like

The methods `from_yaml()` and `load_from_yaml()` are generic and should be defined on a trait like `FromYaml`. These can then get a blanket implementation for anything implementing `serde::Deserialize`. This way any custom extension of `ServerConf` will have that functionality implemented automatically.

Method `to_yaml()` can also be a blanket implementation for anything implementing `serde::Serialize`.

This will make extending the default configuration easier. Another positive side-effect: `serde_yaml` will stay a dependency of Pingora and won’t have to be pulled in for the applications using it.

## Describe alternatives you've considered

It isn’t too hard to reimplement this functionality for custom extensions of course, just unnecessary.