Add coupon discount validator

[stevieraykatz]

We are adding a coupon discount validator which relies on a backend service to sign valid coupons for end users. We leverage EIP-191 signing to validate that the signature is valid for a specific user with a unique coupon id.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[cb-elileers]

Note for posterity: Solady's ECDSA recover function reverts if the signature returns 0, so we don't need to do 0 address validation when setting signer.

Approved review 2367507115 from cb-elileers is now dismissed due to new commit. Re-request for approval.