Option to skip ServiceAccount creation and use an existing one

[miskr-instructure]

It should be possible to configure the observe/agent chart through values.yaml such that it doesn't create the ServiceAccount but tries to bind to an existing one. This would be useful for us because of IRSA in an EKS environment: we need to annotate the ServiceAccount in advance which is not feasible if it is created by the helm release.

The current workaround is using a post renderer to discard the observe-agent-service-account object from the manifest but it's not a great solution for maintainability.

[obs-gh-alexlew]

Thanks for the feature request! You're right, it would add some flexibility to allow binding to existing accounts. We'll open a feature request ticket internally.