Move away from spring.factories for auto-configurations

[mhalbritter]

Using spring.factories to find auto-configuration classes is causing problems with our native work. We initially used it because the code was already available, but we want to offer an alternative in 2.7 and stop using spring.factories for auto-configuration in 3.0.

Task:

• Design and document a file format that can replace spring.factories for auto-configuration
• Update the AutoConfigurationImportSelector and the ImportAutoConfigurationImportSelector to not use spring.factories anymore
• Update documentation tooling in buildSrc to read the new file
• Check with existing IDE and tool vendors to ensure the new format doesn't cause issues

[mhalbritter]

The autoconfigurations are now annotated with @AutoConfiguration and are put in a file named META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration.

The autoconfigurations are listed each in a separate line, comments are supported with the # character, and empty lines are ignored.

See the current documentation.