feat: flatten/slim zksync dependency for easier install and soldeer

[coffeexcoin]

PR-Codex overview

This PR focuses on restructuring the contract libraries and interfaces within the lib/era-contracts directory, updating paths, and adding new Solidity files with interfaces and error handling for the ZKsync ecosystem.

Detailed summary

• Deleted lib/v2-testnet-contractsremappings.txt.
• Updated submodule paths in .gitmodules.
• Added SystemContractErrors.sol with an Overflow error.
• Introduced IAccountCodeStorage interface with multiple functions.
• Created Utils.sol library with utility functions.
• Added Constants.sol with system contract addresses and constants.
• Implemented IAccount interface with transaction validation functions.
• Developed IPaymaster interface for paymaster transaction handling.
• Defined Transaction struct in TransactionHelper.sol for ZKsync transactions.
• Created IContractDeployer interface for contract deployment management.
• Added SystemContractsCaller library for system contract calls with specific flags.

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