API endpoint to get single SSH key for specific user

Description

Create an API endpoint that returns a single SSH key for a given user. Parameters:

id - ID of specified user (integer, required)

key_id - SSH key ID (integer, required)

Background

This is an issue to capture the following MR: !81790 (merged)

The main driver for this is to implement a proper and easy to maintain resource in the GitLab terraform provider.

This this enables GitLab SSH keys to be a Terraform-managed resource.