RPC Sign method : Add a description to explain the purpose of signing transactions

### Is your feature request related to a problem?

_No response_

### Describe the solution you'd like

Add in the sign request object a description

```

{
"serviceName": String, // Service name to use to sign the tx
"pathSuffix": String, // Additional information to add to a service derivation path (optional)
"description": String { // Readable description to explain the purpose of signing transactions (locale + description)
"en": "Readable description in English",
"fr": "Description lisible en français",
},
"transactions": [
{
"type": String, // Type of transaction
"version": Number, // Version of the transaction (used for backward compatiblity)
"data": Object // Transaction data zone (identity, keychain, smart contract, etc.)
}
]
}
```

### Additional context

https://github.com/archethic-foundation/archethic-wallet-client-dart/issues/40

### Epic

_No response_

Locate the RPC Sign method and the sign request object in the TypeScript SDK, then inspect how transaction signing requests are represented and tested. Add the optional localized description shown in the issue and verify that signing requests accept and preserve the English and French descriptions without breaking existing request shapes.

Written by the indexing model from the issue text.