feat(contracts): implement MpcPaymaster for gasless transactions

ERC-4337 paymaster contract that sponsors gas costs for approved MPC
smart accounts, enabling AI agents to execute transactions without
paying gas directly.

Features:
- Owner-managed sponsorship whitelist
- Configurable per-account limits (total and daily)
- Global daily sponsorship cap
- Automatic period reset for daily limits
- Batch sponsorship for multiple accounts
- Full ERC-4337 compliance with validatePaymasterUserOp/postOp
- Stake management for EntryPoint requirements

Author: Kazopl