View Runner IP in the API

Description

As an admin in a managed environment (entreprise), I want to be able to see the runner IP in the runner API, so I can check that they are under control and in a managed space. Proposal gitlab-runner#1771 (closed) was asking for the Ip address in the web pages and it was done, but not in the API. Having it in the API allows to automate a lot of checks.

Proposal

Links to related issues and merge requests / references

The gitlab-runner#1771 (closed) already had the feature, but only in webpage.