[Checklist] Providing Xline client Rust SDK

[bsbds]

Background:

As discussed in #273, we need to provide a Xline specific client SDK for Rust. While some progress has been made on the client, further refinement is required before it can be released as a standalone SDK.

Checklist

• Implement APIs
  • Implement all APIs supported by Xline, maintain the progress in README
• Update README
  • The README should include introduction sections to users, and maintain a section to track the compatible version of Xline
• Add examples to the crate
  • For each API client, there should be an example under xline-client/examples
• Update documents
  • Add detailed description for each method, struct and enum
  • Add examples to API methods
• Release
  • Publish xline-client as a standalone SDK on crates.io