Blend V2 flash-loan callback. Blend has transferred amount of token
(USDC) to this contract. Run both swap legs, transfer total USDC back
to caller so Blend's subsequent Repay can pull it, clear params.
fn exec_op(
env: soroban_sdk::Env,
caller: soroban_sdk::Address,
token: soroban_sdk::Address,
amount: i128,
_fee: i128,
)Stash params + pre-fetch Sushi oracle hints for user. Called by the
Starter before Blend.flash_loan.
No require_auth: anyone could stash params for anyone, but consumption
in exec_op requires Blend to invoke us as from=user, which requires
user's auth on the outer flash_loan. So unauthorized stashing is inert.
fn set_arb_params(env: soroban_sdk::Env, user: soroban_sdk::Address, params: ArbParams)