(feature) Be notified when a flag change

[thomaspoignant]

Requirements

Be notified when a flag change.
Add a system that calls a webhook when a flag is changing.

• The webhook should be configured in the ffclient.Config{} type.
• We can have multiple webhook.
• The webhook is triggered when:
  • Something has changed in the flag (anything).
  • A flag is added to the file.
  • A flag is removed.
• We should not trigger the webhook when we 1st load the file.
• The payload should contain:
  • The name of the current instance using the go-feature-flag.
  • A list of the changes.

A webhook call will look like this:

POST 
{
   "meta": {
     "hostname":"",
     "IP":"0.0.0.0"
   },
   "flags": {
     "deleted": {}, //only flag names
     "added":{}, // full flag object
     "changed":{} //  flag name + differences
   }
}