Pass request headers to the JS enrichment (close #901)

[stanch]

Pass a raw list of headers as the third argument (after event and parameters) to the JavaScript enrichment function.

Example usage:

function process(event, params, headers) {
  for (header of headers) {
    const jwt = header.match(/X-JWT:(.+)/i)
    if (jwt) {
      doSomething(jwt[1].trim())
    }
  }
}

[benjben]

Looks good 👍