Clean Up Leftover Multi-Meta

There is a potential bug in the REST API (identified by @MiguelAxcarHM, and being discussed between @TimothyBJacobs and @kadamwhite here in WordPress core Slack) where updates to meta values can fail unexpectedly because of unrelated other meta.

Our working theory is that this happens when a meta key is registered (using register_meta or register_post_meta) with 'single' => true, but that meta key still has multiple rows in the database as you would expect if it was previously used as 'single' => false. For example,

+---------+---------+---------------------------------------------+---------------+
| meta_id | post_id | meta_key                                    | meta_value    |
+---------+---------+---------------------------------------------+---------------+
|    8898 |    5804 | process_owner                               | 333           |
|   11870 |    5804 | process_owner                               | 333           |
|   13868 |    5804 | process_owner                               | 333           |
|   15748 |    5804 | process_owner                               | 333           |
+---------+---------+---------------------------------------------+---------------+

This plugin provides a CLI command which can be used to iterate through all posts of a post type, check for registered meta for that post, and identify whether any single-registered meta keys have duplicated rows in the postmeta table. The duplicated meta IDs are then cleaned out.

If a meta key has multiple rows for a post with different values, we should leave those alone.

Example command:

wp clean-up-leftover-multi-meta --post-type=my-cpt-name --dry-run