Add featureFlag option for VSD panels

What does this MR do and why?

This change adds the featureFlag option for enabling or disabling a specific product analytics/vsd panel using a development feature flag. The feature flag can be also scoped to a particular project (the analytics pointer project).

Example:

  - title: 'Group usage overview'
    featureFlag: 'my_feature_flag' # new config
    visualization: usage_overview
    queryOverrides:
      namespace: gitlab-org
      filters:
        include:
          - groups
          - projects
    gridAttributes:
      yPos: 1
      xPos: 1
      height: 1
      width: 12

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Create a new value stream dashboard yaml file following this guide: https://docs.gitlab.com/ee/user/analytics/value_streams_dashboard.html#customize-the-dashboard-panels

1. Add the featureFlag option with an existing feature flag (example: product_analytics_admin_settings) to one of the panel config.entry
2. Load the dashboard on the UI.
3. The panel where the feature flag was configured should not be visible.

Related to #448542 (closed)