[PP-2] PostgreSQL: Use native uuid column type instead of varchar for uuid values

Problem/Motivation

Database columns containing UUID values are currently being defined as varchar(128) on all databases. PostgreSQL has a native UUID type, and it would seem sensible to use this, for both storage and data integrity benefits.

Proposed resolution

Ensure the schema definition provided by in \Drupal\Core\Entity\Plugin\Field\FieldType\UuidItem uses the uuid type for PostgreSQL, and falls back to the existing default varchar(128) for all other databases.

Remaining tasks

The following tests are failing as a result of this change:

• Drupal\config\Tests\ConfigDependencyTest
• Drupal\image\Tests\ImageFieldDefaultImagesTest
• Drupal\editor\Tests\EditorFileUsageTest
• Drupal\editor\Tests\EditorFileReferenceFilterTest

User interface changes

None.

API changes

None.