Enable all collateral to be withdrawn

[tomjeatt]

See: https://discord.com/channels/745259537707040778/1138755389001842759

To withdraw all collateral we need to make a different call to the parachain. This may require some UX work as well (e.g. a 'withdraw all collateral' toggle). This has already been implemented on the parachain, so this can already be done from the Polkadot UI.

[tomjeatt]

This requires a lib change. The UI calls the vaults.withdrawCollateral method which takes an amount with a type of MonetaryAmount. To withdraw all collateral, we need to be able to pass in None rather than a MonetaryAmount.