step-certificates: expose Service externalIPs

[phillid]

Description

This adds a new Helm value service.externalIPs to allow setting of step-certificates Service's externalIPs.

This is especially useful when deploying to bare-metal clusters without load balancers.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}