feature: allow multiple api keys in `servingEngineSpec.vllmApiKey`

[kkmarv]

Describe the feature

I want to be able to:

helm install vllmstack vllm/vllm-stack \
  --set servingEngineSpec.vllmApiKey="key1,key2,..."

or

helm install vllmstack vllm/vllm-stack \
  --set routerSpec.vllmApiKey="key1,key2,..."

Why do you need this feature?

From the vllm serve docs I'd expect that it's possible to set servingEngineSpec.vllmApiKey to a comma separated list of API keys. This would allow us to distribute multiple tokens to various stakeholder groups.

Additional context

The current behavior is that while comma separated strings do work but any tokens after the first one are discarded.