[kong] add support for sidecar containers

[dsem]

What this PR does / why we need it:

Adds support for arbitrary sidecar containers to the Deployment resource.

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• PR is based off the current tip of the next branch and targets next, not master
• New or modified sections of values.yaml are documented in the README.md
• Title of the PR and commit headers start with chart name (e.g. [kong])

[CLAassistant]

All committers have signed the CLA.

[rainest]

Sweet! This is a good flexible approach to doing what I currently do with debug branches (see ce1b952 for an example) in a way that's much easier to utilize in practice (forking and adding one's own template changes is no fun).

One minor change request to point users to the relevant K8S object reference doc. Sadly no way to not pin that to a specific version yet, but that's a fairly minor concern--I'm fine pointing that to current latest/v18 and leaving it up to readers to figure out if they need to change to an older or newer version.

[dsem]

Thanks for the feedback and so glad you'll find it helpful too!

[rainest]

Passes all practical testing on my end!

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[hbagdi]

Thank you for your contribution. Please fill out the following form to claim your contributor swag:
https://github.com/Kong/kong/blob/master/CONTRIBUTING.md#contributor-t-shirt.