Expand the capability of handling demand flexibility in different demand sectors

# :rocket:

- [x] Is your feature request essential for your project?

### Describe the workflow you want to enable
I wish the engine could take in demand flexibility for each demand sectors, residential, commercial, industrial, etc., separately rather than aggregating them into one demand flexibility profile on the top of a unified demand profile. In the mean time, flexibility of different demand sectors should be able to have corresponding cost associated with each of them.

### Additional context
From the optimization perspective, specifying demand flexibility by sectors might not make a difference without different costs to dispatch it in different sectors. Before implementing this, we could conduct end-to-end tests (running test scenarios with demand flexibility via PowerSimData given [#590](https://github.com/Breakthrough-Energy/PowerSimData/pull/590) has been merged recently) by aggregating demand flexibility in all sectors into one profile. But eventually, we will need this feature to be able to conduct concrete studies and support ongoing external projects.