feat(rust): Add high-level Wallet wrapper

[rnnpeng]

Description

This PR introduces a high-level Wallet wrapper to the Rust SDK (rust/src/wallet.rs), aiming to bring the Rust developer experience closer to parity with the Python and TypeScript SDKs.

Currently, Rust users must manually construct raw API requests using the generated Client. This change abstracts that complexity by providing a convenient Wallet struct that handles:

• Initialisation with the API Client.
• Simplified method signatures for common operations.

Changes:

• Created rust/src/wallet.rs.
• Exposed the wallet module in rust/src/lib.rs.
• Added unit tests in rust/tests/wallet_tests.rs to verify initialisation and client binding.

Tests

I have verified the changes locally using cargo test.

Method: Wallet::new()
Network: N/A (Unit Test)
Setup: Mocked Client initialisation with dummy credentials

Output:
running 1 test
test test_wallet_initialization ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s

## Checklist

- [ ] Updated the [typescript README](https://github.com/coinbase/cdp-sdk/blob/main/typescript/src/README.md) if relevant (N/A - Rust only)
- [ ] Updated the [python README](https://github.com/coinbase/cdp-sdk/blob/main/python/README.md) if relevant (N/A - Rust only)
- [x] Added a changelog entry
- [ ] Added e2e tests if introducing new functionality (Pending API keys for integration tests)

[cb-heimdall]

🟡 Heimdall Review Status

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