feat: watch-only external on-chain accounts

[shubhamkmr04]

Done so far

• Import a watch-only account from public descriptors (import_watchonly_account)
• Derive receive addresses (watchonly_new_address)
• Read balance, UTXOs, and revealed addresses
• Sync accounts against Esplora (sync_watchonly_accounts)
• Persist each account under its own KVStore namespace — survives restarts
• FFI surface + AccountId type in ldk_node.udl
• Unit tests + regtest integration test (import → fund → sync → restart → intact)