Add $skip_default option to \Drupal\consumers\Negotiator::negotiateFromRequest

Motivation

I have a situation where I need to use \Drupal\consumers\Negotiator::negotiateFromRequest to get the consumer from the request and then act on it. But, if there is no consumer data in the request do something different. Right now, \Drupal\consumers\Negotiator::negotiateFromRequest always returns consumer because it uses whatever is configured as the default if none is found in the request.

Proposed resolution

Add a (bool) $skip_default parameter to both \Drupal\consumers\Negotiator::negotiateFromRequest , and \Drupal\consumers\Negotiator::doNegotiateFromRequest so that when you're invoking the former you can optionally ask it to skip loading the default consumer and instead return NULL .

API changes

This adds a new option to the methods, but shouldn't break any existing implementations.