Backend management

It would be nice to have in the toolbox backend subcommand the procedures to manage the whole backend lifecicle:

• create
• delete
• add metrics
• add methods
• add mapping rule
• add it to a service with a path

cfr.

• /admin/api/backend_apis.json
• /admin/api/services/ {service_id}

  /backend_usages.json

• /admin/api/backend_apis/ {backend_api_id}/mapping_rules.json
  - /admin/api/backend_apis/{backend_api_id}

  /metrics/

  {metric_id}

  /methods.json

• /admin/api/backend_apis/ {backend_api_id}

  /metrics.json