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.

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[brkinney]

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