Reconcile immutable Charge Detail Records against Sessions and model tariff step_size billing rounding using OCPI 2.2.1's CDRs and Tariffs modules

Steps

1. Implement the CDRs module so the CPO POSTs a new CDR to the eMSP's receiver interface after a session ends; the eMSP's response Location header is used for later GET retrieval.
2. Treat CDRs as immutable and billing-final — never PUT/PATCH an existing CDR; if a correction is needed, send a Credit CDR with a negative total_cost referencing the original via credit_reference_id.
3. Implement the Tariffs module so the CPO pushes tariffs to the eMSP via PUT (replace-in-place), or the eMSP falls back to pulling tariffs via GET with date_from/date_to/limit pagination.
4. Structure each Tariff as a list of TariffElements, each grouping price_components (ENERGY priced per kWh, TIME/PARKING_TIME priced per hour with a step_size in seconds, FLAT with no step_size) under shared restrictions.
5. Apply step_size rounding by rounding usage up to the next step (e.g. 42 actual parking minutes billed as 45 minutes at a 15-minute step_size).