Make distributionByTariff data available as totalCost value is not accurate with TimeOfUse

[Simonfitzgerald]

The API totalCost value does not accurately represent the actual cost where time-of-use tarrifs are in use. Evnex describe this as a legacy value, one that appears to use a single tariff where that tariff lacks precision. In Home Assitant time-of-use tariff's can be added in a template as an example and with totalEnergyUsage we can calculate a more accurate cost. Except where there is a mix of solar and grid energy, as we don't know the mix of grid and solar energy. However this is contained in the get session response.

    "totalEnergyUsage": {
        "total": 1041.0,
        "distributionByTariff": {         -- If these values could be surfaced we could work out the true cost ourselves
            "solar": 88.9,                      -- either including solar export as a percieved cost or just a zero value. 
            "flat": 11.1
        },
        "distributionByEnergySource": {
            "cogeneration": 0.1,
            "gas": 0.1,
            "geothermal": 2.4,
            "hydro": 7,
            "localSolar": 88.9,
            "solar": 0.2,
            "wind": 1.3
        }
    },
    "totalCost": {
        "currency": "NZD",
        "amount": 0.0348,   -- This is not a correct value where time of use tariffs are used.
        "distribution": {
            "flat": 100
        }

I am going to attempt this myself, but I am not a Dev so raising it here as a backstop.

Regards