[Phragmens] return election priority of the elected validators

[clangenb]

Motivation

NPOS frontends written in rust would get the ability to show a validator's election priority and also be able to make prognoses about how close a validator is from being elected (I know that the priority can be very dynamic even for minute nomination changes, but from experience it is still a valuable metric).

There is already a W3F funded offline-election-tool written in rust, which does provide valid NPOS solutions. I have forked it and patched the PSDK to also be able to show the election priority.

The patch is very simple, it merely consisted of returning an already computed value, and then ignoring it later. For now, it is a simple hack, but it should be easy to integrate it neatly into the PSDK.

Solution

I propose to improve and integrate the patch upstream, it should not affect any public NPOS related API nor add any computational overhead as the value is computed already anyhow.

[bkchr]

CC @Ank4n @sigurpol