Move away from spring.factories for auto-configuration imports

[mhalbritter]

See #29698

The most important changes are:

• All AutoConfigurations are now annotated with @AutoConfiguration and the registration of those moved from spring.factories to the new file. Together with the changes in the selector this is now a backwards-compatible change. The spring.factories loading can be deprecated now and removed in 3.0.0
• Here's the actual code which does the loading from the new file
• Update of the AutoConfigurationMetadata gradle task
• The documentation has been updated
• To get @ImportAutoConfiguration working, the selector has been updated and the documentation has been updated
• The test slice configuration has been moved from spring.factories to the new mechanism (here's the example for Cassandra)

Task list

• Design and document a file format that can replace spring.factories for auto-configuration.
• Update the existing import to use both spring.factories and the new entries.
• Introduce a new @AutoConfiguration annotation which is meta-annotated with @Configuration. The new annotation will default proxyBeanMethods to false.
• Document the changes in the reference docs
• provide a wiki migration guide
• 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

The following nice-to-have tasks have been identified:

• Develop a migration tool to convert our existing codebase (a tool will be valuable for other project, but we might also be able to use sed)
• Update our annotation processor so the the new auto-configuration file can be auto-generated

[mhalbritter]

I splitted the commits into different issues and commited them directly to 2.7.x branch. This PR is obsolete.