[BUG] `additionalConfig` being used with multiple node pools does not work correctly

[jimdickinson]

What is the bug?

additionalConfig being used with multiple node pools does not work correctly. The order of the node pools matters and configuration is copied forward as they are reconciled.

How can one reproduce the bug?

Make two node pools in the yaml, set additionalConfig on only the first one. Both the first and second node pool will have the configuration set by the first.

What is the expected behavior?

The addtionalConfig from one node pool should not be used on any other.

What is your host/environment?

k8s 1.31

Do you have any screenshots?

none

Do you have any additional context?

I believe the right fix is the code here should make a new map and return that, instead of mutating the first argument.

https://github.com/opensearch-project/opensearch-k8s-operator/blob/v2.8.0/opensearch-operator/pkg/helpers/helpers.go#L152-L160

https://github.com/opensearch-project/opensearch-k8s-operator/blob/v2.8.0/opensearch-operator/pkg/reconcilers/cluster.go#L143

I'll put up a PR to fix the issue.

[prudhvigodithi]

[Triage]
@jimdickinson thanks for the PR #1059, adding @rootxrishabh to please take a look.

[jimdickinson]

@prudhvigodithi any comments on the PR?