feat: implement EthGetBalance and verify against Lotus

[elmattic]

Summary of changes

Changes introduced in this pull request:

• Implement the Filecoin.EthGetBalance RPC method.

| RPC Method                 | Forest              | Lotus               |
|----------------------------|---------------------|---------------------|
| Filecoin.EthGetBalance (4) | Valid               | Valid               |

Reference issue to close (if applicable)

As part of #3639

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.