Implement Get workitems API

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------- COIN API ENDPOINTS -------

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GET /workItems

	PARAMS
		{
			max: int # OPTIONAL, max count of responses to return. Default=100 (Maximum=1000)
			repo: str/list[str] # OPTIONAL
			branch: str/list[str] # OPTIONAL
			type: str/list[str] # possible types [provision, build, test]
			before: ISO 8601 datetime # OPTIONAL, Exclusive. Default=[current datetime]
			after: ISO 8601 datetime # OPTIONAL, Inclusive. Default=[current datetime - 10days]
			status: str/list[str] # OPTIONAL [running, done, failed, cancelled]
		{

	RESPONSE
		{
			more: bool # True if more results than the requested max count exist
			next: str # OPTIONAL, URL-encoded query string to GET /tasks which auto-populates
				  'before' with the last returned task 'started_on' datetime. This effectively allows
				  iterative calling of the /tasks endpoint until one or less results are returned.
			workitems: datetime-sorted list [WorkItemDetails]
		}