Provide a ContextDefinition plugin

[MPParsley]

In #483 we added a block Drupal\og\Plugin\Block\MemberCountBlock that shows the number of members in the current group.

Instead of using dependency injection for the Drupal\og\OgContextInterface we could write a Drupal\Core\Plugin\Context\ContextDefinition plugin so any custom block can use a context_definition to get the current group.