feat(deployment) add template rendering to initContainers

[jeroen-vankelecom]

What this PR does / why we need it:

Allow users to pass helm templates (e.g. for the image name) when definining additional initContainers

Which issue this PR fixes

• fixes Allow templated values in initContainers field #825

Special notes for your reviewer:

None

Checklist

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

[CLAassistant]

All committers have signed the CLA.

[pmalek]

Hi @jeroen-vankelecom 👋

This change looks good, are you still interested in contributing this? If so, then please rebase on most recent main tip and please add an exemplar values.yaml in https://github.com/Kong/charts/tree/main/charts/kong/ci so that the test harness can test this on CI and we can see an actual usage of this.