Add delegation information for validators endpoint: unique delegators (Nova) [medium]

[begonaalvarezd]

Description

Priority: medium

This is part 2 of 2 of a feature we need for delegation information for validators/stakers. The idea of this endpoint is to provide something similar to delegation balance endpoint for validators with the number of unique delegators

The 2 parts are:

1. delegation outputs
2. delegators

Motivation

Provide delegation information for validators

Requirements

Open questions (optional)

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

No.