Create a general purpose `AvailabilityState` interface and refactor
the existing `LivenessState` and `ReadinessState` to use it. A single
`AvailabilityChangeEvent` is now used to carry all availability state
updates.
This commit also renames `ApplicationAvailabilityProvider` to
`ApplicationAvailabilityBean` and extracts an `ApplicationAvailability`
interface that other beans can inject. The helps to hide the event
listener method, which is really internal.
Finally the state enums have been renamed as follows:
- `LivenessState.LIVE` -> `LivenessState.CORRECT`
- `ReadinessState.READY` -> `ReadinessState.ACCEPTING_TRAFFIC`
- `ReadinessState.UNREADY` -> `ReadinessState.REFUSING_TRAFFIC`
See gh-20962
bb79c847b2