Add staker statistics: validators in committee (Nova) [medium]

[begonaalvarezd]

Description

Priority: medium

This is part 1 of 3 statistics we would like to have for Nova. For this statistics, we would like to track how many validators were selected to be part in the commiittee.
Maybe unrelevant detail: afaik nodes usually store the committee for the last 365 epochs.

The 3 parts are:

1. validators in committee
2. candidates
3. total

Afaik, this needs an INX endpoint

Motivation

Improve statistics to accomodate to new IOTA 2.0 features, like staker statistics

Requirements

Open questions (optional)

Are you planning to do it yourself in a pull request?

No.