Deprecate the webhook field to have the field Notifiers instead

[thomaspoignant]

Description

• The config field Webhooks used to notify an API when a flag has changed is too restrictive because we need to have other types of service to call when a flag has changed.

  This PR introduces a new field called Notifiers in place of the Webhooks field to be able to have different types of notifiers when a flag has changed (slack can be a great example).

  Based on that the field Webhooks is deprecated and will continue to be supported in the next versions.
  It will be removed in a future release. (Check migration section bellow to see how to re-configure your webhooks).

• This PR contains also a small change around the HTTPClient used at different place to have a central place where we can configure it.

Migration

If you were using Webhooks before, you should have a configuration like this:

ffclient.Init(ffclient.Config{ 
    Retriever:      &ffclient.FileRetriever{Path: "testdata/flag-config.yaml"},
    FileFormat:     "yaml",
    Webhooks:       []ffclient.WebhookConfig{
        {
            PayloadURL: " https://example.com/hook",
            Secret:     "Secret",
            Meta: map[string]string{
                "app.name": "my app",
            },
        },
    },
}

With Notifiers, your configuration should looks like this now:

ffclient.Init(ffclient.Config{ 
    Retriever:      &ffclient.FileRetriever{Path: "testdata/flag-config.yaml"},
    FileFormat:     "yaml",
    Notifiers: []ffclient.NotifierConfig{
        &ffclient.WebhookConfig{
            PayloadURL: " https://example.com/hook",
            Secret:     "Secret",
            Meta: map[string]string{
                "app.name": "my app",
            },
        },
    },
}

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)

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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