feat: http2 support

[climba03003]

@Uzlopak Sorry for duplication, but it should be how I would like to implement the http2 with native solution.

It still missing test for http2.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Wouldnt it maybe faster if we do it like this?

'use strict'

const status = { ':status': '103' }

function writeEarlyHints (reply, headers) {
  return new Promise(function (resolve) {
    if (reply.raw.socket === null) {
      resolve()
      return
    }
    Object.assign(headers, status)
    reply.raw.stream.additionalHeaders(headers)
    resolve()
  })
}

module.exports = writeEarlyHints

[climba03003]

It should resolve after the headers are added.
Not directly resolved.

[Uzlopak]

We can avoid the value variable.

  for (const key of Object.keys(headers)) {
    if (Array.isArray(headers[key])) {
      message += `${key}: ${headers[key].join(', ')}${CRLF}`
    } else {
      message += `${key}: ${headers[key]}${CRLF}`
    }
  }

How is the performance of for-of-loops?