Implement spring-boot-devtools support

[plaird]

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