Support custom host (instead of '*') for Spring Boot Starter

[DestBro]

Support costume host (instead of '*') for Spring Boot Starter.

Right now there is a way to config only the ports via:

kotlin:
  jupyter:
    kernel:
      ports:
        hb: 9091
        shell: 9092
        control: 9093
        stdin: 9094
        iopub: 9095

For security reasons, I would like to specify a custom host as well, instead of using "*" (which binds to all interfaces). Ideally, it should default to "localhost" to restrict access.