Feat/blacklist

[oscarj007]

The implementation of the blacklist mechanism is complete, covering the following:

1. Updated payment_router/src/lib.rs to include a new Blacklist DataKey in the Soroban contract.
2. Added a new Error::Blacklisted code.
3. Implemented blacklist_address, unblacklist_address, and is_blacklisted functions in the PaymentRouter contract,
   ensuring admin authorization.
4. Integrated the blacklist check directly into the route_payment function, ensuring any transaction to a
   blacklisted recipient fails with Error::Blacklisted.
5. Added comprehensive unit tests in payment_router/src/lib.rs for the new blacklist functionality.
6. Added placeholder admin API endpoints in stellar-payment-platform/src/routes/v1/adminRoutes.js for future
   integration with the smart contract's admin methods.

closes #315

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[Abdulazeem-code]

fix backend build test error