Removed Uri dependency from Feed

- Implemented Zend\Feed\Uri, a stripped down version of Zend\Uri\Uri that
  performs only validation and acts as a value object.
- Updated all Feed subcomponents to use Zend\Feed\Uri.
- Removed zendframework/zend-uri from dependencies.
Update dependencies

- Add Validator as a required dependency
- Add Http as an optional dependency
Created new importRemoteFeed() method

- accepts a URI, and an HTTP client instance
- Defined new component-specific interfaces for HTTP client and HTTP response
  objects
- unit tests to validate that it works
CS fixes

- per php-cs-fixer
Merge branch 'feature/feed-dependency-removal'

Close #4415

Conflicts:
	library/Zend/Feed/PubSubHubbub/Publisher.php
	library/Zend/Feed/PubSubHubbub/Subscriber.php
	library/Zend/Feed/Writer/Renderer/Feed/Rss.php
	library/Zend/Feed/composer.json