chore(shield): add support for rapid-response existing_secret

[francesco-furlan]

What this PR does / why we need it:

Add the following configuration to support rapid-response existing secrets:

features
  respond:
    rapid_response:
      # The name of the existing Rapid Response secret
      existing_secret:
      # The key used in the existing Rapid Response secret for the password (default: password)
      existing_secret_password_key:

Checklist

• Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• Chart Version bumped for the respective charts
• Variables are documented in the README.md (or README.tpl in some charts)
• Check GithubAction checks (like lint) to avoid merge-check stoppers
• All test files are added in the tests folder of their respective chart and have a "_test" suffix