Add sensors for invoice information

[DCSBL]

Depends on #74

3 new sensors are added with cost information about the invoice:

• Previous period
• Current period
• Upcoming period

I am not sure yet how things go when the upcoming period is not yet available, but let's see how it performs :)

[DCSBL]

Can be undrafted when #74 is merged

[DCSBL]

Drafting this until https://github.com/DCSBL/python-frank-energie/issues/50 is addressed

[bajansen]

lgtm