Send the current applicable rate for the interval with the octopus_energy_electricity_current_day_rates event

[silverman10]

Describe the feature

With every octopus_energy_electricity_current_day_rates, along with min_rate and max_rate, send the current/now rate.

Expected behaviour

if the event is triggers at 13:10, payload would look like

...
    - start: "2024-06-28T12:30:00+01:00"
      end: "2024-06-28T13:00:00+01:00"
      value_inc_vat: 0.00441
      is_capped: false
    - start: "2024-06-28T13:00:00+01:00"
      end: "2024-06-28T13:30:00+01:00"
      value_inc_vat: 0.00042
      is_capped: false
    - start: "2024-06-28T13:30:00+01:00"
      end: "2024-06-28T14:00:00+01:00"
      value_inc_vat: 0
      is_capped: false
    - start: "2024-06-28T14:00:00+01:00"
      end: "2024-06-28T14:30:00+01:00"
      value_inc_vat: 0.00462
      is_capped: false
...
  min_rate: -0.02835
  max_rate: 0.291375
  now_rate: 0.00042
  average_rate: 0.07665913
`

### Use Case

Ive been displaying the rates in an apex chart and having the current applicable rate as the header would be useful. Looked at other available entities and none of them gives that information.

### Confirmation

- [X] By submitting this feature request, you agree that you have read the [documentation](https://bottlecapdave.github.io/HomeAssistant-OctopusEnergy) and confirmed it does not already exist
- [ ] I am willing/able to help contribute to the solution of this feature