feat: add rcBlockHash to useRcBlock response

[TarikGul]

Add rcBlockHash to useRcBlock Response

Summary

Adds rcBlockHash field to all useRcBlock=true responses.

Changes

• Updated AbstractController.getHashFromRcAt() to return rcBlockHash alongside existing rcBlockNumber
• Updated all account, block, and pallet controllers to destructure and include rcBlockHash in enhanced responses
• Updated guides: USE_RC_BLOCK_SPEC.md and ASSET_HUB_MIGRATION.md

Example Response

[{
  "at": { "hash": "0x...", "height": "1234567" },
  "free": "1000000000000",
  "rcBlockHash": "0xabcdef...",
  "rcBlockNumber": "20000000",
  "ahTimestamp": "1705123456789"
}]