feat(sdk): add PaymasterClient for gasless transaction support

TypeScript client for interacting with the MpcPaymaster contract,
enabling AI agent wallets to execute gasless transactions.

Features:
- Check sponsorship status and remaining limits
- Add paymaster data to UserOperations
- Estimate max gas costs before submission
- Parse and encode paymasterAndData field
- Utility functions: packGasLimits, packGasFees, createEmptyUserOp

Author: Kazopl