core: session - add all starkAppInit and starkAppExit states to the Session config to make them configurable

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

We have a use case where this can be useful:

The showcase deployed in GithubPages shows immediately the "Initialization failed: could not fetch user profile." dialog since we have no mock backend deployed yet.
In this case we would need to modify the starkPreloadingStateName to show the DEV login page instead.

Expected behavior

The initialization and exit screens should be fully configurable by exposing all the different starkAppInit and starkAppExit states in the Session config passed to the Session Module.

Minimal reproduction of the problem with instructions

Run the showcase in PROD mode without running any local backend.

What is the motivation / use case for changing the behavior?

Fully configurable initialization and exit screens.

[SuperITMan]

In the showcase > app.module.ts, you could remove the condition

if (env === "development) {
  return require(".. /.. /config/json-server/data.json");
}

And set the mockdata with this all the time.