Implement an actual Helm chart

[unflxw]

Our existing Helm chart is just a copy and paste of the existing deployment.yaml for kubectl, with some unrelated broken files alongside it.

Replace this with templates that actually interpolate the values in the chart. Rather than having the release process copy-paste the deployment.yaml from the root into the chart, do the opposite: use Helm's template as the source of truth to generate deployment.yaml from.

Fixes #59.

To do

• Update docs for Helm (see README.md changes)

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[unflxw]

This also changes deployment.yaml and the Helm installation instructions to install it in the appsignal namespace. I think this is a good idea, and shouldn't affect functioning since it's got permissions to all namespaces, but it is a breaking change for people who already installed it (re-applying will create a second set of AppSignal for Kubernetes pods and resources)

[tombruijn]

This also changes deployment.yaml and the Helm installation instructions to install it in the appsignal namespace. I think this is a good idea, and shouldn't affect functioning since it's got permissions to all namespaces, but it is a breaking change for people who already installed it (re-applying will create a second set of AppSignal for Kubernetes pods and resources)

It's probably not possible to warn people this will happen, right? Other than a note in the changelog.

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• This Pull Request has been approved. Can it be merged @unflxw? - (More info)

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