Allow configuration of the init/sidecar container names globally

[dtrts]

Is your feature request related to a problem? Please describe.

I'm currently in an environment where two vault-agents will be running simultaneously. The current set up has the agents selecting mutually exclusive pods. As part of development work within the k8s cluster it is not obvious which agent was responsible for the init/sidecard containers in the target pods. This is because the container names are hardcoded within the agent.

There is an option to use a json-patch annotation to rename the containers. However this is done on the pod and would have the same effect on the containers regardless of which agent was injecting them.

If for some reason a pod was selected by multiple agents this would also prevent name clashes of containers within the pod. (I'm not sure this would even work!)

Describe the solution you'd like
A configuration option for the container names defined here and here which can be set globally and thus confidently reveal the agent which triggered the injection.

Describe alternatives you've considered
I have been active in a closed issue which appears to be requesting the same feature: #290

This issue was closed with a redirect provided to this PR: #291

As mentioned on the other thread this did not resolve my problem in the way I expected.

[dtrts]

cc. @tomhjp