Add @api annotation to block argument marker interface

[Vinai]

Description

Without the @api annotation implementing this interface would require a patch level dependency on magento/framework.
This PR changes that by adding an @api annotation so that only a minor version dependency is required according to http://devdocs.magento.com/guides/v2.2/extension-dev-guide/versioning/dependencies.html

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) NA
• All automated tests passed successfully (all builds on Travis CI are green) NA

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
MAGETWO-88276 has been created to process this Pull Request

[magento-engcom-team]

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

[Vinai]

@magento-engcom-team Thanks for accepting the PR.
When I click the link to https://github.com/orgs/magento/invitation it redirects me to https://github.com/magento. I don't think I have an open invitation.

EDIT: Invitation accepted.