Add new input plugin type: Webhook

[cduez]

Required for all PRs:

• CHANGELOG.md updated
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

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As proposed in #1248, we moved service plugins which listen independently for webhooks into a new plugin type Webhook. This allow us to register each of these plugin under the same http listener.

What do you think?

[sparrc]

IMO the webhook plugin should just be a standard service input plugin.

That way the current and future webhook plugins will just need to be defined within the "webhook" service input plugin. This allows the webhooks plugin to not be an integral part of the telegraf agent, and also still allows for defining webhook services on multiple ports.

ie, config could look something like:

[[inputs.webhooks]]
  address = ":1925"
  [inputs.webhooks.github]
    path = "/github"
  [inputs.webhooks.rollbar]
    path = "/rollbar"

[[inputs.webhooks]]
  address = ":1920"
  [inputs.webhooks.foobar]
    path = "/"

Apologies for misunderstanding your proposal earlier

[francois2metz]

Ahah, It was my original thought too.
That's we pushed the PR early.

Let's do it.