Replace secret vars for arrays

[juliaElastic]

When implementing replacement of secret vars in agent policies here, we left out a scenario when the variables can have array values.

See thread here for more info.

Expected fix: check if vars have array values, replace secrets in them.

To reproduce:

• Add secret: true to the manifest locally:

      - name: api_token
        type: text
        title: API Token
        multi: false
        required: true
        show_user: true
        description: The Alienvault OTX API token
        secret: true

• Add AlienVault OTX integration and populate the API token/key.
• Enroll an agent and check the agent policy sent to the agent.
• The secrets in request.transforms should be replaced.

    streams:
      - id: httpjson-ti_otx.threat-a520c8d4-1c8e-48c6-8525-d046ab1b8bda
        data_stream:
          dataset: ti_otx.threat
          type: logs
        config_version: '2'
        interval: 5m
        request.method: GET
        request.tracer.filename: ../../logs/httpjson/http-request-trace-*.ndjson
        request.url: 'https://otx.alienvault.com/api/v1/indicators/export'
        request.timeout: 30s
        request.transforms:
          - set:
              target: header.Content-Type
              value: application/json
          - set:
              target: header.X-OTX-API-KEY
              value: '${SECRET_0}'

[jen-huang]

Moving into current sprint as I think @michel-laterman has some time. Michel, would you please take a look at this?