feat: add vault transfer (deposit/withdraw) support

[lihaostar]

Summary

• Add VaultTransfer action struct with RMP+Agent signing support
• Add HttpClient::vault_transfer() for depositing/withdrawing USDC from vaults
• Add vault_transfer example for SDK usage
• Add hypecli vault command with deposit and withdraw subcommands

The usd field is serialized as a plain integer in micro-USDC units (1 USD = 1,000,000), consistent with other RMP-signed actions like UpdateIsolatedMargin. The public API accepts Decimal in USDC and converts internally.

Usage

SDK

// Deposit 100 USDC into a vault
client.vault_transfer(&signer, vault_address, dec!(100), nonce, true).await?;

// Withdraw 50.5 USDC from a vault
client.vault_transfer(&signer, vault_address, dec!(50.5), nonce, false).await?;

Example

# Deposit 100 USDC
cargo run --example vault_transfer -- --vault 0x1234... --operation deposit --amount 100

# Withdraw 50 USDC
cargo run --example vault_transfer -- --vault 0x1234... --operation withdraw --amount 50

hypecli

# Deposit (private key)
hypecli vault deposit --private-key <KEY> --vault 0x1234... --amount 100

# Withdraw (private key)
hypecli vault withdraw --private-key <KEY> --vault 0x1234... --amount 50

# Deposit (keystore)
hypecli vault deposit --keystore <NAME> --vault 0x1234... --amount 100

# Withdraw (keystore)
hypecli vault withdraw --keystore <NAME> --vault 0x1234... --amount 50

[lihaostar]

@ifdario could you please help review this PR? thanks.