[feature]: `FundChannel`: add `push_asset_amount` option

[ZZiigguurraatt]

In FundChannel and OpenChannel we can set a push_sat to push an initial amount of sats to the channel peer when opening a channel. We do not have such an option for assets. It would be nice to add a push_asset_amount option to FundChannel to get this same kind of feature for the assets in a channel. This would be useful for all the same reasons we have the option push_sat.