feat: add isPlatformAccount to PaymentInstructions and remove PaymentAccountOrWalletInfo refs (#128)

### TL;DR

Refactored payment account schemas to simplify the inheritance structure and added platform account identification.

### What changed?

- Removed the `PaymentAccountOrWalletInfo` base schema and its discriminator pattern
- Simplified inheritance by having payment account types directly reference their base account info
- Replaced `enum` with `const` for account types and asset types to improve type safety
- Relocated the `ClabeAccountInfo` schema to a more logical position in the schema hierarchy
- Added `isPlatformAccount` boolean field to `PaymentInstructions` to distinguish between platform and customer accounts
- Moved `NgnAccountInfo` schema to maintain better organization

Author: pengying