Fix running shell in the Spring Boot applications

[piotrooo]

Partially resolves: gh-1203

After the removal of DefaultShellApplicationRunner in this commit: ff73bdc, Spring Boot applications no longer have an entry point for shell runners.

This causes shell programs not to run.

[fmbenhassine]

@piotrooo Thank you for the PR! But I think there is deeper issue: command discovery is still broken. In the hello world example, spring-shell-starter brings spring-shell-standard which is supposed to add basic commands, but that is not the case. I mean with this PR, the hello world sample now works again and the command hello is recognized, but it's impossible to exit the shell.. exit and quit are not registered.

Moreover, I do not understand why ShellRunner#run returns a boolean and not void as it was before. The javadoc mentions Return value indicates if run operation happened and no further runners should be used. but in my opinion it is not the ShellRunner's repsonsbility to decide the flow of execution (ie if another runner should run or not). More explicitly, If I want to implement a custom runner, I don't know if I should return true or false in my custom implementation. In others words, having to do the following seems awkward to me:

boolean run = runner.run(args.getSourceArgs());
if (run) {
   return;
}

I think we need to fix that before proceeding as well.. Thoughts?

[piotrooo]

I've opened a discussion at #1209. It should be more convenient.