feat: enabled usage of private key with docker autoscaler (#1232)

## Description

This PR enables users to explicitly allow using the private key used to
authenticate the runner manager with the workers when using the
`docker-autoscaler` option.

This is required in order for the manager to be able to connect with the
workers running Windows, as noted here:
https://gitlab.com/gitlab-org/fleeting/plugins/aws#default-connector-config

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Co-authored-by: Matthias Kay <[email protected]>
Co-authored-by: Matthias Kay <[email protected]>

Author: zillemarco