Support GE spot as price source

[Heronimonimo]

With all recent issues it seems that relying on a single source is flakey.

The GE Spot Integration combines multiple sources and unifies the output formats: https://github.com/enoch85/ge-spot

I think this could solve a lot of issues for Nordpool diff and extending its reach to a wider audience.

[jpulakka]

Definitely looks cool. Especially

Unified output - Standardizes to 15-minute intervals (96 data points per day)

sounds exactly what is needed. While nordpool_diff supported fallback between two sources (custom-components/nordpool and JaccoR/hass-entso-e) and that actually worked quite well before #33 , the challenge with both of them is that there's no documentation of what exactly they give out; that had to be guesstimated experimentally, and unsurprisingly, has not been predictable when entering the 15-minute MTU era.

[jpulakka]

There are "Today interval prices" and "Tomorrow interval prices" attributes in many (most? all?) entities provided by GE-Spot. The attributes look like this

Today interval prices
- time: '2025-12-05T00:00:00+02:00'
value: 0.0787
raw_value: 0.0787
- time: '2025-12-05T00:15:00+02:00'
value: 0.0777
raw_value: 0.0777
- time: '2025-12-05T00:30:00+02:00'
value: 0.0762
raw_value: 0.0762
...
- time: '2025-12-05T23:45:00+02:00'
value: 0.066
raw_value: 0.066
Tomorrow interval prices
- time: '2025-12-06T00:00:00+02:00'
value: 0.0685
raw_value: 0.0685
...
- time: '2025-12-06T23:45:00+02:00'
value: 0.0127
raw_value: 0.0127

= Good! What's the difference between "value" and "raw_value", apparently VAT enoch85/ge-spot#40 .

This should be reasonably easy to integrate.