Add attribute for providing Classpath index file in executable jar

[plaird]

Some newer features of Spring Boot executable jar file format are not available in the Bazel Spring Boot jar rule yet.

• Classpath index file
• Layer index file

https://docs.spring.io/spring-boot/docs/current/reference/html/appendix-executable-jar-format.html#executable-jar-war-index-files-classpath

This issue is only for the classpath index file. The layer index file is more involved and will be covered in another issue.

I don't think we should do anything magical here (i.e. auto-generating these files). Just allow the user to specify existing workspace files as attributes in the rule.

[plaird]

We just had an internal use case come up that requires the classpath.idx file, so we are moving this up into the next minor release train (1.1).

[plaird]

This is implemented as you can provide the classpath.idx file and it is inserted correctly into the executable jar, but I am not convinced yet that it actually works. Perhaps I misunderstand the Spring Boot feature.

I also need to document this.

[plaird]

Docs were provided in PR #70, closing.