Add support of Exec Dashboard configuration file parsing

Summary

as part of #383559 (closed) we need to parse file .gitlab/analytics/dashboards/value_streams/value_streams.yaml which shares schema with analytics dashboards

Configuration schema

Proposed schema of data attribute

data:
  type: metrics_comparison
  namespace: {GROUP_OR_PROJECT_NAMESPACE}

README.md

Add documentation information about .gitlab/analytics/dashboards/value_streams/value_streams.yaml to README.md