Add subcommand info

[amotl]

Hi again,

this patch will add the grafana-wtf info subcommand. It will display the Grafana version and global statistics about many entities. Please let me know you if you are missing anything important. Per-entity statistics will be tackled with #18.

Data for the statistics slot is coming from Grafana's /admin/stats endpoint. Data for the summary slot is computed by grafana-wtf itself.

With kind regards,
Andreas.

/cc @jangaraj

Example

$ grafana-wtf info --format=yaml

grafana:
  version: 8.3.2
  url: http://localhost:3000
statistics:
  activeAdmins: 1
  activeEditors: 0
  activeSessions: 0
  activeUsers: 1
  activeViewers: 0
  admins: 1
  alerts: 0
  dailyActiveAdmins: 0
  dailyActiveEditors: 0
  dailyActiveSessions: 0
  dailyActiveUsers: 1
  dailyActiveViewers: 0
  dashboards: 1
  datasources: 3
  editors: 0
  monthlyActiveUsers: 1
  orgs: 1
  playlists: 0
  snapshots: 0
  stars: 0
  tags: 2
  users: 1
  viewers: 0
summary:
  annotations: 0
  dashboards: 1
  datasources: 3
  folders: 1
  notifications: 0
  organizations: 1
  snapshots: 0
  teams: 0
  users: 1
  dashboard_panels: 11
  dashboard_annotations: 1
  dashboard_templating: 5