[SDK-2273] Add onRedirecting login hook

[stevehobbsdev]

Changes

This PR adds a hook function to WebAuth.login and WebAuth.passwordlessLogin that would enable a developer to handle the event where a login ticket has been retrieved, just before redirecting away to the /authorize endpoint.

The hook is a function that can be specified, where the done function that is given must be called when the hook is complete, to let the SDK know that it can resume with authentication.

Usage example:

webAuth.login({
  username: 'username',
  password: 'password',
  onRedirecting: function(done) {
    // Your hook code here
    done();  // call this when finished
  }
}, function(err, res) {
});

Same for passwordless:

webAuth.passwordlessLogin({
    connection: 'email',
    verificationCode: 'code',
    email: 'email',
    onRedirecting: function(done) {
      // Your hook code here
      done();   // call this when finished
    }
  },
  function (err, res) {
  }
);

This PR also includes:

• Doc comments updates
• Readme formatting updates across the board
• A new playground script to quickly test out this feature for login and passwordless

Testing

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All tests and linters described in the Develop section run without errors