Add action to test kustomize config validity

[ACLay]

Refs #408

Mirror's the config repo's jenkins job to dry-run apply our kubernetes config and catch any syntax errors before they make it into the main branch. I've set the kubeconfigs up as repo secrets for now. Will look at moving them to organisation ones once I'm confident everything's set up properly.

Examples of failed and successful runs can be seen in the last few commits, after I worked out the bugs in the action.

[ACLay]

There seems to be some variability in how shells handle errors in loops. Locally, this if statement was needed to catch the non-zero exit code from a failed kubectl apply ... and force the loop to exit with an error, but the action itself just exits on the failed kubectl apply .... Keeping this check here means the code can still be used locally, and we're covered incase the action's environment changes.