Expose `BookedDate` and `StartDate`

[iaindillingham]

We need to make Appointment.BookedDate and Appointment.StartDate available through patients.with_gp_consultations for an upcoming study that @LisaHopcroft is coordinating.

At present, only Appointment.SeenDate is made available through this function, so we need to add a new argument that allows a user to select the column they wish to query. Precedents:

• Several functions, such as patients.address_as_of, accept a returning argument that allows a user to select a column.
• patients.with_these_decision_support_values accepts an algorithm argument, which allows the user to select an algorithm (spoiler: only the Electronic Frailty Index is available).

Lisa and I have undertaken some curation work on these columns; a notebook has been released to the appointments-short-data-report workspace. However, I see that we restrict the values in the status column to these

cohort-extractor/cohortextractor/tpp_backend.py

Lines 2105 to 2113 in faca1e5

	valid_states = [
	AppointmentStatus.ARRIVED,
	AppointmentStatus.WAITING,
	AppointmentStatus.IN_PROGRESS,
	AppointmentStatus.FINISHED,
	AppointmentStatus.PATIENT_WALKED_OUT,
	AppointmentStatus.VISIT,
	]
	valid_states_str = codelist_to_sql(map(int, valid_states))

from these

cohort-extractor/cohortextractor/tpp_backend.py

Lines 3813 to 3828 in faca1e5

	class AppointmentStatus(enum.IntEnum):
	BOOKED = 0
	ARRIVED = 1
	DID_NOT_ATTEND = 2
	IN_PROGRESS = 3
	FINISHED = 4
	REQUESTED = 5
	BLOCKED = 6
	VISIT = 8
	WAITING = 9
	CANCELLED_BY_PATIENT = 10
	CANCELLED_BY_UNIT = 11
	CANCELLED_BY_OTHER_SERVICE = 12
	NO_ACCESS_VISIT = 14
	CANCELLED_DUE_TO_DEATH = 15
	PATIENT_WALKED_OUT = 16

in cohort-extractor. Our curation work hasn't explored the status column, so I'd tentatively advocate allowing a user to disable this restriction.