[Backport] Add 'const' type support to layout arguments

[IgorVitol]

Original Pull Request

#15058

Description

For now in Magento we can not use xsi:type="const" in layout arguments. But it looks like such ability can be very useful. Moreover such code already exists in Magento core, we just need to include/enable it.

Manual testing scenarios

1. Add argument with "const" type to any block via layout arguments. For example like this:

<referenceBlock name="product.info.sku">
    <arguments>
        <argument name="my_const_value" xsi:type="const">Magento\Catalog\Api\Data\ProductAttributeInterface::ENTITY_TYPE_CODE</argument>
    </arguments>
</referenceBlock>

2. Verify that it is correctly passed to block constructor. Like here: http://prntscr.com/jf7qa0

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 @miguelbalparda, thank you for the review.
ENGCOM-1624 has been created to process this Pull Request

[magento-engcom-team]

Hi @IgorVitol. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.