Implement filtering by country for custom power plants

[ekatef]

Describe the feature you'd like to see

The workflow currently fetches all the data put into custom_powerplants.csv, also for the cases when custom data are provided for several countries but only a few of them are included into the model.

A workaround is enhancing powerplants_filter (thanks @yerbol-akhmetov for the hint!) but that's a rather hacky approach. A more convenient way would be implementing filtering by country into build_powerplants script.