[feat] helm-charts: add support for secrets

[ecniiv]

What problem does this PR solve?

The secrets field defined in the DorisCluster specification was not being handled when deploying via Helm chart. This PR add support for the secrets field in the Helm chart template and values file for all component specs.

Related issue: #242

Added secrets configuration section with documentation and examples for each component spec:

• Default empty array: secrets: []
• Example commented configuration showing how to use secrets:

feSpec:
  secrets:
    - secretName: my-secret
      mountPath: /etc/doris