introduce snapshot testing for the chart

[jlandowner]

Summary of changes:

Hi, thank you for the useful and stable chart.
I would like to introduce snapshot testing for the chart project.
It allows to you to detect any change of the chart for common usecases.
You can check the impact on common usecases on Git before release.

Here is the examples.

Confirm not to break any existing feature.

Detect a broken change.

Update the snapshots by a single command

This way is adopted in Kong chart project. https://github.com/Kong/charts/tree/main/charts/kong/ci
It is also good to do snapshot testing in CI but it's not in this PR so far (but Kong does!)

ref:
https://github.com/Kong/charts/blob/main/.github/workflows/release-pr.yaml#L208
https://github.com/Kong/charts/blob/main/Makefile#L70

Please feel free to try it!

Checklist:

• I updated the artifacthub.io/changes annotation in Chart.yml according to the documentation
• Optional: I updated in README.md the Helm Values