feat(core): Provide OperationResultSource from Client methods

[kitten]

Summary

Return a new OperationResultSource from all Client methods (which replaces PromisifiedSource on shortcut methods). This allows not only toPromise() to be called, but it can also be used as an awaitable PromiseLike and has a .subscribe(onResult) method aliasing the subscribe utility from wonka.

Set of changes

• Rename PromisifiedSource to OperationResultSource
• Add then and subscribe methods to OperationResultSource
• Use withPromise on executeRequestOperation (i.e. making the above available on all client methods returning sources)