Improve ways in which configuration property metadata default values can be provided

The hope is that we could avoid problems like https://github.com/spring-projects/spring-boot/issues/28037. In that case there's a List<String> property with a default value created with new ArrayList<>("…"). The annotation processor can't extract the default value so we rely on an entry in the manual metadata for it. For such a property that doesn't have an entry in the manual metadata, it'd be good if the annotation processor could fail or generate a warning.

Start by examining Spring Boot's configuration metadata annotation processor and how manual metadata supplies default values. Compare the case described in issue 28037, especially the List default that cannot be extracted. Done should mean properties whose defaults cannot be extracted fail or produce a warning when no manual metadata entry exists.