Allow to inject parameters into webhook handlers

[gr2m]

For the upcoming octokit we need to be able to inject a pre-authenticated octokit client to the webhooks handler, see the Webhooks section

const webhooks = client.webhooks({secret})
webhooks.on('issue.opened', ({id, name, payload, client}) => {
  return client.rest.issues.createComment({body: 'Hello, World!'})
})
require('http').createServer(webhooks.middleware).listen(3000)

The client parameter in the code snippet above needs to be injected, it is not part of @octokit/webhooks API. We need to be able to run code after a webhook is received, but before it is passed to the webhook handlers.

One approach would be to implement middleware similar to what Express does: https://expressjs.com/en/guide/using-middleware.html. There is a related issue on Probot: probot/probot#598

An alternative would be to leverage the hook API that we already use in @octokit/rest and that will be part of octokit: https://github.com/octokit/rest.js#hooks.

[gr2m]

I will use the before-after-hook like we already do in @octokit/rest. I will allow to pass an optional hook option to the constructor so that the upcoming octokit client can use the same hook instance for both, request and webhook hooks.

const WebhooksApi = require('@octokit/webhooks')
const OctokitRest = require('@octokit/rest')
const webhooks = new WebhooksApi({
  secret: 'mysecret'
})

webhooks.hook.before('webhook', (options) => {
  options.client = new OctokitRest()
  options.client.authenticate({
    type: 'token',
    token: process.env.GITHUB_TOKEN
  })
})

webhook.on('foo.bar', options => {
  // options.client is set now
})

webhooks.receive({id: '123', name: 'foo', payload: {action: 'bar'}})
// logs "Webhook received: foo.bar (id: 123)"

[gr2m]

The current .transform option might actually be good enough for this, it was created for Probot for exactly this reason: https://github.com/probot/probot/pull/335/files#diff-687648a33bac039342470366b890374cR15

[gr2m]

The current .transform option might actually be good enough for this

It is.