[CMS] Add ability to load block and pages by identifiers

[okorshenko]

Description

As a Magento develop I would like to have a @api interface to get CMS pages and blocks by identifiers and store id.

Existing API

Existing API of the CMS module provides an ability to:

• get page/block by ID (BlockRepositoryInterface::get, PageRepositoryInterface::get)
• search page/block by search criteria (BlockRepositoryInterface::search, PageRepositoryInterface::search)

Solution

1. Introduce BlockManagementInterface and PageManagementInterface to provide an ability get CMS pages and blocks by identifiers and store id.

namespace Magento\Cms\Model;
/**
 * @api
 */
interface BlockManagementInterface
{
    /**
     * Load Block data by given Block identifier.
     *
     * @param string $identifier
     * @param int|null $storeId
     * @return \Magento\Cms\Api\Data\BlockInterface
     */
    public function getByIdentifier($identifier, $storeId = null);
}

namespace Magento\Cms\Model;

/**
 * @api
 */
interface PageManagementInterface
{
    /**
     * Load Page data by given Page identifier.
     *
     * @param string $identifier
     * @param int|null $storeId
     * @return \Magento\Cms\Api\Data\PageInterface
     */
    public function getByIdentifier($identifier, $storeId = null);
}

2. Provide default implemetation for the new interfaces.
3. If store id is NULL, use current store ID.

Note: All new @api classes must be covered with the tests

Related issues:

1. There is no ability to pass store ID during getting CMS page/block by identity. #6570
2. CMS Page and Block - Load by identifier and store id - fix issue 6570 #7417
3. [Cms] Added a getByIdentifier() method to the BlockRepository #9163
4. [Cms] Added a getByIdentifier() method to the block reposistory #8858

[piotrkwiecinski]

@okorshenko if no one will grab it before next week you can assign it to me. I should have some time to work on it next weekend.

[piotrkwiecinski]

@okorshenko I'll have time to work on it. If you could assign it to me.

[damiantomczak]

@damiantomczak