add method to start h2 webserver console inside reactive environment

[atomfrede]

This is a preparation for jhipster/generator-jhipster#13699 to start the h2 console without/outside a servlet context so we can have it when using reactive. Micronaut blueprint is doing the same as it runs on netty by default and no servlet context is available.

PR in the main generator will follow.

[pascalgrimaud]

I don't know if it's difficult to create test for that ?

[atomfrede]

Let me check, but I guess it should be quite easy, starting the webserver and checking if there is something up and running (if we have a valid properties file here).