Allow more flexible time calculation for time based cover

**Describe the problem you have/What new integration you would like**
Covers (Rolladen) in Germany need some time to open, they have a special pre-open state where the blinds move away from each other to let a little bit of light through. Only after that the blinds start to open.
With that the 50% position will never be at the halfway point of the window.

**Please describe your use case for this integration and alternatives you've tried:**
I initially wanted to implement the Tasmota way of calculating the position via time, Tasmota has additional calibration points (like a halfway point). For this to be implemented an additional component needs to be created.

I thought it might be much easier if the time calculation could be accessible via a lambda to be modified from the template itself.

**Additional context**

No files, tests, or entry points are named in the request. Start by comparing the current time-based cover calculation with Tasmota's calibration-point approach, then determine how a template lambda could customize it; done means supporting covers with a pre-open state and non-linear position timing.

Written by the indexing model from the issue text.