Record GDPR deletion date for donors

[data-sync-user]

As a Mozilla legal representative dealing with a GDPR deletion request from a customer who is a MoFo donor, I need CTMS to (1) record that the deletion was requested and (2) record a date of four years from present, so that after the legally-mandated four year waiting period expires (as required for donor data), either a legal representative or an automation can find this customer's data and delete it.

Acceptance criteria: a datetime field has been added to the CTMS schema, we have a means to manually set it when needed.

┆Issue is synchronized with this Jira Story