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
