feat: implement decide API (#634)

* Implement createUserContext

* Clean up

* Incorporate comments

* Implement decide api

* Add decide api unit tests

* add flag to DECISION_NOTIFICATION_TYPES

* Store defaultDecideOptions as object with boolean values

* Start adding decide api unit tests to optimizely client

* Finish decide api with empty default decide options unit tests

* Change alldecideOptions to object instead of an array

* Update user context unit tests

* Fix unit test

* Address comments

* Add tests when both DISABLE_DECISION_EVENT and EXCLUDE_VARIABLES are provided

* Log warning when unrecognized decide option provided

* Remove redundant featureFlag

* Rename optimizely_decision.ts. to index.ts

* Fix notification data object

* Remove sourse field from decision notification number

Author: yavorona