Implement spring-boot-devtools support

The devtools feature of Spring Boot is intended to speed up development. Details:

• https://docs.spring.io/spring-boot/docs/current/reference/html/using.html#using.devtools
• https://docs.spring.io/spring-boot/docs/current/reference/html/using.html#using.running-your-application.hot-swapping
• https://www.baeldung.com/spring-boot-devtools

This will take some investigation, but I believe we need to do the following:

• when building the jar/war packages, we need to exclude devtools by default for security reasons (don't want them in production)
• have an attribute to include devtools for jar/war (see excludeDevtools property in Maven)
• have a devtools variant of bazel run that uses the exploded form of the Spring Boot launcher
  • this is key, because devtools does not work when launched from jar, which is how we launch today
• wire up all the locations that Bazel puts compiled resources into the devtools launcher

Start by reading the linked Spring Boot devtools documentation and the repository's existing Bazel rules for jar/war packaging and the current Bazel run path. Identify how compiled resources are placed and how the Spring Boot launcher is invoked. Done means devtools is excluded from packages by default, can be explicitly included, and a devtools run variant uses the exploded launcher with all compiled resource locations wired in.