This commit adds the `AutoConfigurationImportsAnnotationProcessor` to
the `spring-boot-autoconfigure-processor` annotation processor
module. When added to a project build, the annotation processor will
generate the
`org.springframework.boot.autoconfigure.AutoConfiguration.imports`
file automatically from `@AutoConfiguration`-annotated classes. It
also applies the annotation processor to the Spring Boot build.
Closes gh-31228
Implements a new AutoConfigurationLoader, which loads
auto-configurations from a file in META-INF/spring-boot.
Adapts the AutoConfigurationImportSelector to use the new loader.
Adapts the ImportAutoConfigurationImportSelector to use the new loader.
Adapts the metadata plugin in the build to additionally load the
auto-configurations from the new file.
Updates the documentation for auto-configurations and test slices.
Closes gh-29872
Scott Frederick
Moritz Halbritter
Andy Wilkinson
Phillip Webb