Support Secret References in Subscription Recipients

[yanp]

Currently we can reference secrets in service configurations using the $secret-name syntax, but it is not available for subscription recipients. This makes it difficult to manage recipient information (like private Slack channels or distribution lists) through K8s secrets.

Current Behavior

• Service Configurations: Support referencing secrets using $secret-name syntax.
  Example:

  service.slack: |
    token: $slack-token

• Subscriptions: Do not support secret references. Recipient information (e.g., Slack channels, email distribution lists) must be hardcoded.
  Example:

  subscriptions: |
    recipients:
      slack: private-channel-name  # Hardcoded
      email: team-list@company.com  # Hardcoded

Desired Behavior

• Extend secret reference functionality to subscription recipients.
  Example:

  subscriptions: |
    recipients:
      slack: $slack-channel
      email: $team-email

Use Cases

• Storing private/sensitive recipient information (e.g., Slack channels) in secrets.
• Managing different recipients per environment using secrets.
• Centralizing recipient management alongside other configurations.