Implement dep bundles for Spring Boot starters

[plaird]

As a SpringBoot rule user
I would like to have an easy idiom for including an open source Spring Boot starter (e.g. jetty, web)
Such that I do not have to list all the transitive deps for that starter in my BUILD file
Such that the dep list corresponds with the version of SpringBoot I have chosen

Acceptance:

• double down on our current approach, or research and switch to something else
• create/use a tool that can create a bundle for each open source starter at each spring boot version
• publish those bundles somewhere in this repo

Background:
Internally at Salesforce, we have internal Bazel BUILD packages that wrap each open source starter that we use (a small subset of the ones in open source) for a variety of reasons. Those packages list the transitive deps for the SpringBoot starter they each wrap. We also standardize on one Spring Boot version at a given time. So our internal use case is simpler than the general public and is easily curated by hand.

But the general public will use a wider variety of starters, and could be on any of the public Spring Boot releases. We have a crude solution in place for this open source distro here:
https://github.com/salesforce/bazel-springboot-rule/tree/master/tools/springboot/import_bundles

But we should solve this in a more maintainable and comprehensive way.
rules_jvm_external uses a different scheme with maven_install.json files, perhaps consider that. https://github.com/bazelbuild/rules_jvm_external

[plaird]

This was finalized in #67

We are doing option 1 above: switch to something else. We are going to rely on maven_install for this functionality. The PR has the details.