Prototype externalization of Spring Boot CLI properties

DO NOT MERGE - FOR REVIEW PURPOSES ONLY
/cc @dsyer @snicoll This is a partial implementation. I didn't convert all CLI auto-compilation units. But enough to get a cross section. The test cases all pass.

* Move key properties of Spring Boot CLI to mappings.properties.
* Scan for classpath*:mappings*.properties files, so allow overrides/plugins

This provides the means for external parties (JetBrains, start.spring.io) to consume the metadata associated with auto-configuration of Spring Boot CLI apps.