feat: add functions that will wrap observables w/ skywait

[Blackbaud-ColbyWhite]

it's common to want to wrap an observable in the wait service, particularly when making an http call. i.e. you want to show the blocking spinner while a backend call is happening. this makes that paradigm simple and avoids people having to duplicate that pattern throughout their code.

expected usage:

waitSvc.blockingWrap(http.get('url')).subscribe(doStuff);

credit to Nicklaus Glyder for the idea.

[Blackbaud-SteveBrush]

Thanks for the contribution, @Blackbaud-ColbyWhite! One quick comment about backwards compatibility.

[Blackbaud-SteveBrush]

Docs issue: blackbaud/skyux2-docs#592