Call webhook when a flag changed

[thomaspoignant]

Description

This PR adds the possibility to call some webhooks when a flag is changed.
All the webhooks are called in parallel with a POST request with the format bellow.

We can signed the request using HMAC/sha-256 with the secret passed in the configuration.

{
  "meta":{
      "hostname": "server01"
  },
  "flags":{
      "deleted": {
          "test-flag": {
              "rule": "key eq \"random-key\"",
              "percentage": 100,
              "true": true,
              "false": false,
              "default": false
          }
      },
      "added": {
          "test-flag3": {
              "percentage": 5,
              "true": "test",
              "false": "false",
              "default": "default"
          }
      },
      "updated": {
          "test-flag2": {
              "old_value": {
                  "rule": "key eq \"not-a-key\"",
                  "percentage": 100,
                  "true": true,
                  "false": false,
                  "default": false
              },
              "new_value": {
                  "disable": true,
                  "rule": "key eq \"not-a-key\"",
                  "percentage": 100,
                  "true": true,
                  "false": false,
                  "default": false
              }
          }
      }
  }
}

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-comptible and/or changes current functionality)

Closes issue(s)

Resolve #16

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the Readme
• I have followed the contributing guide

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