feat(core): add app.addSetup for asynchronous configuration steps

[vonagam]

Connected to #2500 and contains small fixes from #1611.

Adds app.addSetup() which allows adding asynchronous configuration/initialization steps.
Those will run with services setups during app.setup() call.

Services use this method to add their own setup calls to queue,
so one can add a step that runs before or after a specific service setup.

This is for cases when you don't have an appropriate service to implement setup method in.

Will add tests if that sounds as a reasonable addition.

[marshallswain]

I like this idea. It nicely solves the concerns you had in #2500 (comment). Looks good to me.

[vonagam]

I will close this one since recent hooks for setup/teardown from #2585 should do the trick.

But will note that setting this._isSetup = true should be done before services begin to register, not after, otherwise dynamically registered services from some setup will not get their own setup called. (It was fixed in #1611, but then #2585 put it back because it was based on a code before the fix.)

[daffl]

Ah yes, I meant to get back to ask if that new one may not cover everything but I'm hoping it will suffice. Ugh, I thought I kept your _isSetup change in there because that made sense. Will have to revert that part.