Add category children depth parameter

[paliarush]

As a Magento developer, I want to control the depth of the returned children for the requested category so that my queries can be more performant.

Acceptance criteria

1. Depth = 0 returns all children (default)
2. Depth = 1 returns only immediate children
3. Depth = 2 returns only immediate children and their children; and so on
4. Children are returned in a flat list (depth can be determined by "level" field)
5. Changes are covered with web API functional tests. See \Magento\GraphQl\Catalog\ProductViewTest::testQueryAllFieldsSimpleProduct as an example.

[dmytro-ch]

For now, the category children depth is being calculated recursively by \Magento\CatalogGraphQl\Model\Category\DepthCalculator. So the resulting depth value is based on the specified children structure in request.

[paliarush]

Current issue becomes irrelevant after changes done in #102