Add ObserverInterface to the api

[fooman]

Description

Creating an observer that uses ObserverInterface should not trigger a patch level dependency on magento/framework.

Fixed Issues (if relevant)

N/A

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
MAGETWO-88146 has been created to process this Pull Request

[magento-engcom-team]

@fooman thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[orlangur]

It would be good to add a note also that observers creation should be avoided whenever possible in favor of plugins-interceptors.

[Vinai]

Why? There are plenty of places where an event observer is better suited than a plugin.

[orlangur]

Mainly because "There can be only one" intention.

The only valid place for event dispatching I heard of are some controllers in MSI project.

All other places in current code which seem better suited by observers is just unrefactored legacy which is doing to much. In code written with plugins in mind there is no need in event dispatching at all.

[Vinai]

Lol, there is soooo much legacy code that will never be rewritten because it would break so many extensions. Maybe in Magento 3. But for Magento 2 I don't consider that realistic.