Allow object-based plugin definitions to be processed in DefaultPluginManager::findDefinitions()

Problem/Motivation

\Drupal\Core\Plugin\DefaultPluginManager::findDefinitions() checks for a 'provider' key in the plugin definition array, and removes the definition if the provider does not exist.

However not all plugin definitions are arrays. It attempts to handle this by casting the object to an array, but that only works if there is a public $provider; property in the definition class, which is not ideal.

Proposed resolution

Expand \Drupal\Component\Plugin\Definition\PluginDefinitionInterface.
Previously discussed was adding a new interface, and deprecating PDI.
However, multiple issues want to expand PDI, we can't just introduce a single new interface that replaces it. We'd end up with a confusing circular dependency of which interfaces to use when and which would be deprecated.

@catch cited that problem, and https://www.drupal.org/core/d8-bc-policy → , to suggest that we just expand PDI directly

Remaining tasks

User interface changes

API changes

Data model changes