[ERC Registry] Add ContractScanner service class to handle fetching contracts from mirror node

[quiet-node]

Problem

According to the design document, a new ContractScanner service class is needed to handle fetching contracts from the mirror node.

Solution

Implement a ContractScanner service class under tools/erc-repository-indexer/src/services/contractScanner.ts. The class should include:

• A fetchContracts() method to retrieve all contracts from the mirror node.
• A fetchContractByteCode() method to fetch detailed contract objects using contractId.

The ContractScanner class should also be tested using the Jest testing framework.