Support relative redirect_uris

[aarefiev]

Problem:
When you develop a real project like web-server, you have multiple versions of it:

• development (it can be localhost),
• stage/testing server,
• production server

These servers have their own URLs and they can be changed in the future. According to documentation, you should pass URL in redirect_uris, but you can't specify it in code. You can do it with using environment variables:

const client = new identityIssuer.Client({
  client_id: 'identity-client-id',
  client_secret: 'identity-client-secret',
  redirect_uris: [
    `${process.env.MYSELF_URL}/cb`,
  ],
  response_types: ['code'],
});

but in this case, you should change this variable every time when you want to change the URL. If you forget, an error will occur.

My suggestion:
Use relative URL in configuration:

const client = new identityIssuer.Client({
  client_id: 'identity-client-id',
  client_secret: 'identity-client-secret',
  redirect_uris: [
    '/cb'
  ],
  response_types: ['code'],
});

and resolve it in Strategy, before calling the authentication request, like this:
https://github.com/jaredhanson/passport-oauth2/blob/v1.5.0/lib/strategy.js#L145-L147