Add optional annotations to secrets

[brkinney]

Description: Add optional annotations to be passed for secrets.

My team uses a mutating webhook to inject secrets on create. We utilize annotations as part of this webhook which this chart does not currently support.

Testing: This is an optional value, and tested that no examples show any changes when rendering. I also tested rendering with the below test value file to verify my changes show up correctly. I did not think this change warranted a new example to be created.

agent:
  controlPlaneMetrics:
    etcd:
      secret:
        create: true
        annotations:
          testAnnotation: etcdSecret

secret:
  annotations:
    testAnnotation: secret

Documentation: Updated values schema.

Note:
I went with an if conditional in the templates because I saw it was done this way in the serviceAccount.yaml file:
https://github.com/signalfx/splunk-otel-collector-chart/blob/main/helm-charts/splunk-otel-collector/templates/serviceAccount.yaml#L20-L23

Afterwards I noticed that the service.yaml uses with instead:
https://github.com/signalfx/splunk-otel-collector-chart/blob/main/helm-charts/splunk-otel-collector/templates/service.yaml#L17-L20

I don't know if there is a preference but I can change if requested.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[brkinney]

I have read the CLA Document and I hereby sign the CLA

[atoulme]

Please see the failing build:

FAIL: A new changelog file in .chloggen/ or entry in CHANGELOG.md is required for this commit due to:

• Updates to files under 'helm-charts//templates'
• Updates to Chart.yaml files
  Please run "make chlog-new" for normal PRs or "make chlog-update" for PRs creating releases.
  make: *** [Makefile:115: chlog-validate] Error 1
  Options:
• Add a Changelog Entry: Please add a '.yaml' file to the './.chloggen/' directory detailing the changes in your commit. See 'CONTRIBUTING.md' for guidance on creating this entry.
• Skip the Changelog Check: If your changes don't necessitate a changelog entry or it's a minor chore, you can skip this check by:
  1. Adding '[chore]' to the title of the pull request
  2. Applying the 'Skip Changelog' label to the pull request.

[brkinney]

@atoulme I have added a changelog file to the PR, but I'm unsure of what to put in the issue field. I have a support case number, but I'm unsure if I should be referencing that in the changelog, or if there is supposed to be a github issue linked to this.

[atoulme]

@atoulme I have added a changelog file to the PR, but I'm unsure of what to put in the issue field. I have a support case number, but I'm unsure if I should be referencing that in the changelog, or if there is supposed to be a github issue linked to this.

you can use this PR number. I have gone ahead and added it.

[atoulme]

LGTM. I have asked for a second review from the team.