[helm-chart-ci] Clean up duplicated code for helm chart CI #803
Description
There are multiple places to detect changed helm charts in the CI job.
It's good to implement it in a common function.
https://github.com/opea-project/GenAIInfra/blob/main/.github/workflows/pr-chart-e2e.yaml#L43
https://github.com/opea-project/GenAIInfra/blob/main/.github/workflows/pr-chart-e2e.yaml#L47
https://github.com/opea-project/GenAIInfra/blob/main/.github/workflows/push-release-charts.yaml#L36
https://github.com/opea-project/GenAIInfra/blob/main/.github/workflows/push-release-charts.yaml#L51