Support `WebSocketProvider`

[SeiyaKobayashi]

📝 Overview

At this moment, only HTTPProvider is supported by the SDK. Supporting WebSocketProvider would be beneficial to developers, as some utility functions (e.g., watching contract events) could be implemented more efficiently using WebSocketProvider.

🖋 Details

• Modify SDKClient class to support WebSocketProvider as a provider option.
• Refactor any related code.
• Add unit tests for all the newly-added functionalities (targeted branch coverage = ~90%)

🍭 Sub-Tasks

N/A

📚 References

• WebSocketProvider
• Example WebSocket RPC endpoint