Add Secret resource into Helm chart to allow passing sensitive values to config via environment variables

[TheoBrigitte]

PR Description

This PR introduces a new Secret resources into the Helm chart which is used to store sensitive values as environment variables and populate them into the Alloy container to be read within the Alloy config using the env directive.

This allow for sensitive values to be stored in a Secret rather than in the ConfigMap or the Pod definition.

Notes to the Reviewer

I did my best to add some meaningful examples on how those secret values are defined and used from the config, but I am open to suggestions.

I would also appreciate some guidance on how to update the Helm chart tests.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[clayton-cornell]

ping @grafana/grafana-agent-maintainers for a review. No input for doc in the PR in it's current state.

[wildum]

@petewall pinging you on this PR because it's about the helm chart

[petewall]

Personally, I don't know if I see the value for this, since this chart already has a envFrom, which lets you already do this:

envFrom:
- secretRef:
  name: mySecret

The only difference is that this chart would create a secret for you, which, I guess is helpful. It feels like this chart doing too much, IMO.