Replace hardcoded ERC-20 ABI with JSON file import and eliminate magic numbers

[ongrid]

Currently, the ERC-20 ABI is hardcoded in the Python consts with magic numbers and partial dictionary structures that are difficult to maintain and extend.

See:

• https://github.com/ongrid/intent-gate/blob/d97c5904d80121beae31fadeff2d2b6f79e60c28/app/evm/erc20_service.py#L14
• https://github.com/ongrid/intent-gate/blob/d97c5904d80121beae31fadeff2d2b6f79e60c28/app/evm/const.py#L5C1-L5C21

Issues:

1. Hardcoded ABI: ABI definition is embedded in Python code without clean structure of artifacts
2. Incomplete ABI: Only includes balanceOf function and Transfer topic, missing other standard ERC-20 functions (like allowances) and events (approvals). See: Check ERC-20 token approvals from skeeper to balance manager #19
3. Maintainability: Adding monitoring of new contracts require adding new magic numbers without clear structure See Replace individual balanceOfcalls with a single Multicall contract call #16