Update `ConfigurationMetadataAnnotationProcessor` to include the
'defaultValue' of a property when possible. For example the
'defaultValue' or 'server.port' is '8080'.

Default values are detected by inspecting the field assignments of
@ConfigurationProperties items. In order to detect field values some
internals of the Java compiler are used. To save a dependency on
'tools.jar' internal javac classes are accessed using reflection.

See gh-1001