Make config_provider a reusable abstraction for other packages with a yaml-config

It would be nice if we could share some logic with for example:

* https://github.com/dart-lang/jni_gen/pull/32

Currently the confg_provider is tightly coupled with ffigen config, but it has some reusable abstractions:

```dart
Specification(
requirement: Requirement.no,
validator: booleanValidator,
extractor: booleanExtractor,
defaultValue: () => false,
extractedResult: (dynamic result) =>
_excludeAllByDefault = result as bool,
),
```

Everything besides `extractedResult: ...` in this snippet is reusable.