Add drush parameter to retain entity id

#8 didn't work for me.

I have the exact use case as #6, and we're using D 9.5.9 and version 2.0.0 of DCD.

My steps:

1. Migrate the D7 to fresh D9 site
2. drush config-set "system.site" uuid [Redacted]
3. drush migrate:upgrade --blah --blah
4. drush su (sync UUIDS via a module)
5. drush cim
6. drush dcde menu_link_content (run on prior migrated site using the same menus)
7. drush dcdi --include-id (back to the original site on the above steps)

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1011' for key 'PRIMARY': INSERT INTO "menu_link_content" ("id", "revision_id", "bundle", "uuid", "langcode") VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placehold
  er_4); Array
  (
      [:db_insert_placeholder_0] => 1011
      [:db_insert_placeholder_1] =>
      [:db_insert_placeholder_2] => main
      [:db_insert_placeholder_3] => [redacted]
      [:db_insert_placeholder_4] => und
  )

If you want another use-case for this, parent-child relationships in taxonomy terms, which get exported as IDs not UUIDs. These also present a second challenge because the parent may not be imported before the child. I currently solve this by importing terms twice, which resolves the issue.