Support deprecation / refactoring of configuration properties

[tobiasschaefer]

As requested by @graemerocher I'm filing an issue, see also #4958

Configuration properties work well to support type safe classes representing the configuration.

However, I'm missing:

• marking configuration properties as deprecated - maybe log a warning if they are used
• refactoring configuration properties: Support a new and deprecated structure in parallel after renaming packages or properties. For example I might want to rename "x.y.z" to "x.z":

This is the existing structure:

@ConfigurationProperties( "x")
public interface Configuration {

    @NotNull
    Y getY();

    @ConfigurationProperties("y")
    interface Y {

        @Bindable(defaultValue = "false")
        boolean isZ();
    }
}

This would be the new structure but it misses "y" completely:

@ConfigurationProperties( "x")
public interface Configuration {

    @Bindable(defaultValue = "false")
     boolean isZ();
}

I would be nice to support x.z but fallback to the value x.y.z if it is set.