Add sensor for total cost

[DCSBL]

Depends on #74

We now have 3 'Total costs' sensors:

• (new) Expected costs this month → e.g. €90,12 for this month
• Expected monthly costs until now → e.g. €45,06 if you are halfway the month
• Actual monthly costs → e.g. €40,42 if you are halfway the month and used less than expected, nice!

Maybe we have to tweak the names a bit 🤔 (also see #77)

[DCSBL]

Can be undrafted when #74 is merged

[bajansen]

lgtm