Track payment-processor-specific contact IDs
PayPal, for example, has a unique 'payer_id' for each donor. We could store that in a table similar to civicrm_payment_token, with foreign keys to civicrm_contact and civicrm_payment_processor, plus a varchar 255 field for the payment_processor_contact_id.
This could assist in figuring out which contact an incoming payment is from.
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