Add APP_INITIALIZER token handler

[BrunnerLivio]

I'm submitting a...

[ ] Regression 
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

Nest does not evaluate the order of the initalization of a provider. It would be useful to load certain providers on application initialization, blocking the initialization of the normal providers in case an APP_INITIALIZOR provider uses a promise.

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Expected behavior

Providers which uses the APP_INITIALIZER token should be executed when the application is being initalized.

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

Could be useful to register routes on initialization with #1438

Blockers: #1517

Others:
APP_INITIALIZER Angular Docs

[kamilmysliwiec]

In most cases, async providers + onModuleInit should be enough. Nonetheless, APP_INITIALIZER might be useful if we want to provide a way to register something once all modules + providers are initialized (but application didn't bootstrap yet).

[kamilmysliwiec]

I think that now when we have async providers, onModuleInit, onApplicationBootstrap + we're also tracking onApplicationListen, we dont need to introduce another hook.