Add priority validators list to the power shaping params

[p-offtermatt]

Problem

It is currently possible to either allowlist or denylist validators.
However, it is not possible to prefer certain validators (while still allowing other validators to opt in).

Feature description

We have a "prioritization" feature implemented.
The feature consists of a new list of validator addresses, just like the already existing whitelist and blacklist.
When choosing which validators validate a chain, we prefer to use those validators, as long as they have opted in.
So the rough logic for the validator set construction in power mixing is:

• Get all validators
• Filter out opted-out validators
• Filter out validators on the blacklist/not on the whitelist
• Among remaining validators, take the priority validators
• If these are not enough to fill the validator set size yet, fill up with remaining other validators