[#689] add tx hash to the ada-holder/get-current-delegation

[jankun4]

Extend the endpoint by providing both drepHash and drepView, and tx hash of the delegation

• Add
  ada-holder/get-current-delegation now returns json with drepHash, drepView and txHash

Checklist

• related issue
• My changes generate no new warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the changelog
• I have added tests that prove my fix is effective or that my feature works