Implement `emerald_sign`

Method which allows sign arbitrary message.

```
The sign method calculates an Ethereum specific signature with: sign(keccak256("Ethereum Signed Message: " + len(message) + message))).
Parameters

Address - 20 Bytes - address.
Data - Data which hash to sign.

params: [
"0xcd2a3d9f938e13cd947ec05abc7fe734df8dd826",
"0x5363686f6f6c627573" // Schoolbus
]

Returns

Data - Signed data.
```

Start by locating the existing account and signing entry points in the repository, then compare their APIs with the Ethereum-specific message format described here. Done means an emerald_sign method accepts the stated address and data parameters and returns the expected signed data for the Schoolbus example, with coverage added wherever existing signing behavior is tested.

Written by the indexing model from the issue text.