[Thanos] Use existing secret for thanos objstore config

[samuel-begin]

Q	A
Bug fix?	no
New feature?	yes
API breaks?	no
Deprecations?	no

What's in this PR?

Added new key in objstore section

# Existing secret containing the configuration
objstoreSecretOverride: "my-storage-config"

This setting affect the creation of the secret and their references in bucket, compact and store deployments.

It is empty by default so there are no breaking changes

Why?

In order to use the same secret for thanos and prometheus charts, we must provide a way to get the objstore config form an existing secret. Prometheus is already setup that way, so this PR is a natural extension which completes the set.

Checklist

• User guide and development docs updated (if needed)
• Related Helm chart(s) updated (if needed)

[tarokkk]

The intent is great but usually adding some existing resource to a Chart is done by adding overrides.

In this PR I would love to see

objstoreSecretOverride: ""

If it exists we can mount that secret and use the name from the variable and we don't need a bool variable for checking if we want to use it.

{{- if .Values.objstoreSecretOverride }}
  secretName: "{{ .Values.objstoreSecretOverride }}"
{{- else }}

[samuel-begin]

Thank you @tarokkk

Coming from the stable helm repo, It's the first time I see an override pattern mentioned. Usually, its all about existing resources (secrets, pvcs, configmaps, etc).

That being said, I fitted the changes to your standards and updated the doc accordingly

[tarokkk]

Thank you for the change!

[ahma]

Thanks @samuel-begin LGTM