Implement paymaster flow

[b1m0n]

Please check if what you want to add to go-u2u list meets Contribution guidelines, maintainers note and Quality standard.

• Prerequisites: https://github.com/unicornultrafoundation/go-u2u/pull/92
• References: This PR refer to issue number no https://github.com/unicornultrafoundation/go-u2u/issues/94
• Description:
• [X] TransactionHelper, IAccount, IPaymaster smart contract interfaces and simple implementations for testing purposes
• [X] Implement the paymaster flow first to serve AA gas sponsoring in the future

[b1m0n]

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