[FEATURE] Subscription Contracts

[YoshihitoAso]

1. Is your feature request related to a problem? Please describe.

• We want to realize subscription contracts for tokens on the blockchain.

Requirements

• The "holder" of S-token is obligated to deposit the payment to the "payment agent" on the subscription payment date.
• If the S-token "holder" makes the payment by the due date, the "issuer" of the S-token will give the holder reward tokens.
• If the S-token "holder" fails to make the payment by the due date, the S-token "issuer" will confiscate the S-token.

2. Describe the solution you'd like

Spec

• The "Issuer" outsources the collection of subscription fees to the "payment agent". The "payment agent" is obligated to notify the Issuer of the results of the collection. (Off-chain contractual obligations)
• The "Issuer" registers the "Payment Agent" with the subscription contract.
• The "Payment Agent" records in the subscription contract the holders who have successfully collected the payment and the holders who have failed to collect the payment.

[YoshihitoAso]

Since there is not much need for this feature at the moment, we will not develop it in this project.