Creates an unsigned Avalanche P-Chain import transaction (ImportTx) that imports AVAX previously exported from
the C-Chain. This is step 2 of the C-to-P transfer flow and should be called after the C-Chain export transaction
is accepted.

The custodian supplies the P-Chain wallet that owns the exported atomic UTXO and may provide exportTxHash to
import the UTXO from one specific C-Chain export. If exportTxHash is omitted, CDAO may import currently
available C-Chain atomic AVAX UTXOs for the wallet, subject to backend policy.

After signing, call /avalanche/transactions/prepare, then /avalanche/transactions/submit to dry-run or
broadcast the import through the P-Chain API. After the import is committed, the AVAX is available on the P-Chain
and can be used for validator staking or delegation.