Issue #1896076 by EclipseGc, fago: Added Contextual Plugins and supporting code.

core/lib/Drupal/Component/Plugin/Context/Context.php

.../lib/Drupal/Component/Plugin/Context/ContextInterface.php

core/lib/Drupal/Component/Plugin/ContextAwarePluginBase.php

...b/Drupal/Component/Plugin/ContextAwarePluginInterface.php

...ib/Drupal/Component/Plugin/Exception/ContextException.php

core/lib/Drupal/Core/Plugin/Context/Context.php

core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php

...stem/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php

.../system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php

...n_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php

...Plugin/plugin_test/mock_block/MockComplexContextBlock.php

..._test/Plugin/plugin_test/mock_block/MockUserNameBlock.php

...st/Plugin/plugin_test/mock_block/TypedDataStringBlock.php