Allow IBuildEngine Extension Point

[benvillalobos]

Issue tracking discussion that started here: #6174 (comment)

Issue Description

Adding new information the task can access from the build engine is cumbersome. Current process:

1. Create IBuildEngineX
2. Add one property
3. Modify TaskHost and OOPTaskHost to implement the interface.

Potential Path Forward

1. Create an IBuildEngineX that contains a Dictionary<string, object> where object is serializable.
2. Mihai suggested an IBE with an abstract class called EngineServices.

[Forgind]

@marcpopMSFT suggested we should put this off until mid-April at the earliest.

[rainersigwald]

@ladipro you're basically doing this in one of your outstanding PRs right?

[ladipro]

Affirmative. I wasn't aware of this one, I like the suggested name EngineServices.