Haxe/hxnodejs externs for the agentkeepalive npm library

Tested using agentkeepalive version 4.1.3

Example:

import js.npm.agentkeepalive.*;

final agent = new HttpAgent({
  maxSockets: 100,
  maxFreeSockets: 10,
  timeout: 60000,
  freeSocketTimeout: 30000
})

final httpsAgent = new HttpsAgent({
  maxSockets: 100,
  maxFreeSockets: 10,
  timeout: 60000,
  freeSocketTimeout: 30000
})