policiesApi : modify the schema of retirementCapital->projectedRetirementBenefits into Get policy response

[stadja]

Description
Currently, when requesting a policy response (GET /policies/{policyId}), the API schema states that the value of retirementCapital->projectedRetirementBenefits should be an array representing the projected retirement benefits for a series of retirement ages.

State of the Art Observations
We asked 3 pension funds to implement this API. None of them could provide us with an array of projected retirement benefits. Instead, they only provided the "real" projected retirement benefits corresponding to the client's official retirement age.

Suggested Change
I propose modifying the specification to return not an array, but just an object containing the projected retirement benefits for the client's official retirement age. If the request includes a specification in simulationData to retire before or after the retirement age (see issue 22), it should only return the resulting projected retirement benefits.