Fix TypeScript related issues

[dkundel]

This PR fixes different TypeScript related binding issues found in #374, #376 and by @nash-md. Specifically this fixes:

• Exporting TwiML constructors correctly as twilio.twiml.VoiceResponse etc.
• Making all options in AccessTokenOptions optional to not force the user to provide all if one is present
• Setting the algorithm argument in toJwt for AccessTokens to optional since a default value is present
• Add allowIncoming as new option for VoiceGrantOptions
• Marking the RequestValidatorOptions values as optional

Script used to verify changes

import twilio = require('twilio');

// creating a client
const client = twilio(
  process.env.TWILIO_ACCOUNT_SID,
  process.env.TWILIO_AUTH_TOKEN
);

// create TwiML
const myTwiml = new twilio.twiml.VoiceResponse();
myTwiml.say({}, 'foo');
myTwiml.toString();

console.log(myTwiml.toString());

// create AccessToken
const accessToken = new twilio.jwt.AccessToken(
  process.env.TWILIO_ACCOUNT_SID!,
  process.env.TWILIO_API_KEY!,
  process.env.TWILIO_API_SECRET!,
  {
    ttl: 360,
  }
);

// create Grant
const grant = new twilio.jwt.AccessToken.VideoGrant();
grant.toPayload();

const AccessToken = twilio.jwt.AccessToken;
const voiceGrant = new AccessToken.VoiceGrant({
  incomingAllow: true,
  outgoingApplicationSid: 'someSid',
});
voiceGrant.toPayload();

accessToken.addGrant(voiceGrant);
accessToken.addGrant(grant);

const token = accessToken.toJwt();
console.log(token);

// verify requests

import express = require('express');
const app = express();

app.get('/', (req: express.Request, res: express.Response) => {
  const isRequestValid = twilio.validateExpressRequest(
    req,
    process.env.TWILIO_AUTH_TOKEN!,
    { url: process.env.TWILIO_WEBHOOK_URL! }
  );

  if (isRequestValid) {
    res.send('Yay');
    return;
  }

  res.send('Nay');
});

[Hamoonist]

this commit just saved me! thanks!