driver informs solver about flashloan hints

[MartinquaXD]

The driver needs to inform the connected solver about the flashloan hints attached to an order (i.e. extend API schema).
This is important for a few reasons:

1. the driver needs to be aware that that particular order incurs greater gas costs due to the additional flashloan wrapper call
2. the trader might not have the required sell_token balance for the order but that's irrelevant if the necessary funds can get flashloaned into the settlement
3. depending on which flashloan provider is chosen (e.g. balancer v2, uniswap, aave) it may influence which AMMs can be used to route the liquidity

Especially for reason 3 it's also important to let solvers return in their solution which flashloan provider, token and amounts they would like to use (i.e. extend API schema). This information should be optional for backwards compatibility. If they don't return any information the driver will just pick the information provided by the user in the appdata field while encoding the transaction.