Add a new interest rate tier to an existing purse.

This endpoint allows Partners to add Interest Rate Tiers to an existing primary and savings purses.

Notes:
• The interestYieldStartDate will be set to the current date, if you are adding an interestRateTier to the purse for the first time.
• If the interestYieldStartDate is not provided, the accountCycleDay will be on or after the current date of the request.
• If the purse already has an interestRateTier applied, then the interestYieldStartDate will be set to the next anniversary of the billing cycle.
• The next anniversary of the billing cycle will be returned as the accountCycleDay in the POST /enrollments, GET /accounts, and GET /enrollments endpoint responses.
• The interestYieldEndDate of the previous Interest Rate Tier on the purse is automatically set to the day before the new Interest Rate.
• If the purse has no Interest Rate Tiers applied yet, and autoAlignStartDate is False then the interestYieldStartDate is set to the current date of the request, otherwise it is set to the date provided in the request. On the new additional tiers, when the autoAlignStartDate is True, the interestYieldStartDate is set to start on the next accountCycleDay (Bill Cycle Date).