Make ServiceManager dependency optional in Feed component

This PR makes the ServiceManager dependency in the Feed component optional.

- Creates `Zend\Feed\Reader\ExtensionManagerInterface`, defining required capabilities for an extension manager; `ExtensionManager` implements this interface now.
- Creates `Zend\Feed\Writer\ExtensionManagerInterface`, defining required capabilities for an extension manager; `ExtensionManager` implements this interface now.

If desired, you can substitute your own implementations using:

```php
Reader::setExtensionManager($customImplementation);
Writer::setExtensionManager($customImplementation);
```