Store Subscriber ID as Usermeta

[kimcoleman]

Is your feature request related to a problem? Please describe.
In order to accurately update the subscriber information if the WordPress user profile updates, we need to know the ID of the subscriber in ConvertKit.

The tag a subscriber endpoint we use at membership checkout returns the ID of the newly added subscriber: https://developers.convertkit.com/#tag-a-subscriber

Describe the solution you'd like
We should add a function to update usermeta when the API returns the data from ConvertKit with the Subscriber ID.

Additional context
Doing this is a prerequisite for allowing profile update (email address) changes on the WP side to reflect in ConvertKit.

Note: In reading the ConvertKit API docs it doesn't look like there is a way to retroactively ping for the Subscriber ID. You can call all subscribers (https://developers.convertkit.com/#list-subscribers), so it is possible we could eventually write a one-time script to run and update usermeta with subscriber ID. Some would be unable to be updated though if the user email isn't a match.