Inbound Agent support

[vikrantsnirban]

Enabling Inbound Agent to avoid SSH Connectivity under controlled environment.

Testing done

Installed Plugin to Jenkins and able to create Inbound Agent on-demand by running pipeline with provided label.

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[cotman]

This would be a big boon for us, as we have a Jenkins master in AWS - at the moment we're having to launch GCE nodes with public IPs and use SSH, which isn't ideal from a security point of view - if inbound agent were supported then we could switch to nodes with private IPs (with a lockdown of the inbound agent port on the master to the Cloud NAT IP of the GCP VPC used for the nodes).

Is there a plan on the table to get this into the formal release of the plugin?