Add Solana Asset to Cold Vault

Initiates the creation of a Solana Nonce account for a cold (offline) account, enabling offline transaction signing. This is required before a cold vault can send Solana transactions.

The operation creates a durable nonce account on Solana which replaces the recent blockhash requirement, allowing transactions to be signed offline and submitted at any later time.

A hot (online) account with SOL balance is required to pay for the nonce account creation (rent-exempt balance + fees).