Motivation
In certain cases we want to run update hooks that need to happen after updatedb and config:import have ran. E.g to update content after new fields have been added to a content type. Or to update module configuration that do not use drupal's configuration api and as such their configuration is not captured by the config management system.

Proposed changes
Drush provides a hook we can implement to facilitate such changes which runs when invoking drush deploy or drush deploy:hook. See:

• https://www.drush.org/latest/commands/deploy_hook/
• https://github.com/drush-ops/drush/blob/master/tests/functional/resources/modules/d8/woot/woot.deploy.php
• Add new deploy command. Performs site deployment. drush-ops/drush#4359

Alternatives considered
BLT's native "post-config-import" command.

• Not every project is running BLT and customers adopting BLT may already be used to running their drush deploy hooks since that's a more prevalent Drupal tool in the community.

Testing steps

• Implement a deploy hook as shown in the 2nd bullet point above.
• Apply the patch from this PR.
• Run blt drupal:update

Related issue: #4522