Include "owner" entity key and use user module's EntityOwnerTrait [#3417760]

Problem/Motivation

Storage entity has it's own implementation of Drupal\user\EntityOwnerInterface that does not use EntityOwnerTrait.
In addition to code duplication, this causes with modules like single_content_sync (I'm sure there are others) that rely on existence of "owner" entity key

Proposed resolution

Add "owner" key to the Storage entity definition
Use user module's EntityOwnerTrait to implement EntityOwnerInterface

Remaining tasks

Write the patch

User interface changes

No user changes

Issue fork storage-3417760

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3417760-include-owner-entity changes, plain diff MR !11
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