feat: enabled usage of private key with docker autoscaler

[zillemarco]

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

[github-actions]

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[zillemarco]

@npalm / @kayman-mk I can confirm that with these changes I am able to connect our runner manager (Unix) to the workers (Windows) (issue reported here: #1172 (comment)).

I tested this on our environment, using my fork as the module's source.

[kayman-mk]

Thanks for enable the module to use Windows.

[kayman-mk]

To test the changes it's needed to set up an environment that uses a Windows AMI for the runner workers, and ensure that the manager is able to connect to the instances.