snowbridge: improve destination fee handling to avoid trapping fees dust (#5563)

On messages Ethereum -> Polkadot Asset Hub: whether they are a token
transfer or a `Transact` for registering new token, make sure to handle
unspent fees, rather than trapping them.

This PR deposits them to Snowbridge's sovereign account on Asset Hub.

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Co-authored-by: command-bot <>
(cherry picked from commit 62534e53)