Creates an unsigned Avalanche P-Chain validator staking transaction (AddPermissionlessValidatorTx) for a
CDAO-managed validator. The custodian supplies the account, funding P-Chain wallet, stake amount, staking
duration, and optional reward addresses. CDAO selects the validator NodeID and BLS proof-of-possession from
backend configuration; those values are returned in transactionSummary for custodian verification before signing.

The returned unsignedTxHash is the digest the custodian signs offline. The returned unsignedTxSerialized is
an immutable representation of the unsigned P-Chain transaction. Any change to amount, duration, reward owner,
delegation fee, or selected validator requires creating a new unsigned transaction and collecting a new signature.

Use /avalanche/transactions/prepare to attach the custodian signature, then /avalanche/transactions/submit
to dry-run or broadcast the signed transaction to the P-Chain. rewardAddress receives the validator's own
validation rewards. delegationRewardAddress receives delegation-fee rewards earned from delegators; if omitted,
it defaults to rewardAddress.