[FSSDK-10643] make event processor injectable

[raju-opti]

Summary

• Modify the instantiation API so that the user can inject an event processor directly. Also expose createStaticEventProcessor factory method.

Test plan

• New tests has been added and relevant tests has been refactored

Issues

• FSSDK-10643

[coveralls]

coverage: 88.866% (-0.3%) from 89.201%
when pulling 676129a on raju/event-processor
into 1d814a2 on master.

[junaed-optimizely]

LGTM! Have some queries -

• I was going through the TDD before reviewing this and I find createQueuingEventProcessor which is missing from this commit. Are you planning to do that on the future? Or you are happy with the current createEventProcessor as it is.
• I am seeing some commented code and unused imports across the platform specific files. I have just put "Cleanup required" for each. Pardon me if I have missed any to mark.

[mikechu-optimizely]

WIP still reviewing

[mikechu-optimizely]

Overall, I like where this is going. This is a gigantic PR (likely out of necessity). I'm guessing you have other PRs planned.

I would rather not block progress, but please review the suggestions.