Add an EntityOwnerTrait to standardize the base field needed by EntityOwnerInterface

Problem/Motivation

This was raised as a review point in #2784921-135: Add Workspaces experimental module:

+++ b/core/modules/workspace/src/Entity/Workspace.php
@@ -0,0 +1,223 @@
+      ->setDefaultValueCallback('Drupal\workspace\Entity\Workspace::getCurrentUserId')
...
+  /**
+   * Default value callback for 'uid' base field definition.
+   *
+   * @see ::baseFieldDefinitions()
+   *
+   * @return int[]
+   *   An array containing the ID of the current user.
+   */
+  public static function getCurrentUserId() {
+    return [\Drupal::currentUser()->id()];
+  }

Node and Media also have this, this is the 3th occurance of this exact code + docblock in core, should we open a followup to figure out if it makes sens to move this to a common class, or a trait?

Proposed resolution

Add a EntityOwnerTrait, similar to EntityPublishedTrait.

Remaining tasks

Do it.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.