Public method to transfer native tokens from the contracts

[akolotov]

After addressing Alternative receiver in ERC20-to-NATIVE mode and Alternative receiver in NATIVE-to-ERC20 mode it will be more easy to send tokens from a contract on one chain to an EOA on another chain.

But the bridge contracts do not provide such functionality so far - the payable fallback method cannot be used by contracts since only 2300 gas will be provided for its execution whereas it consumes much more.

Currently the only thing that is doing by the fallback method is a call for the internal nativeTransfer method (native-to-erc20, erc20-to-native). In order to allow the contracts to use the bridge it is necessary to make it payable and public.

[akolotov]

Since the contract will be able to send tokens through the bridge it is required to handle a case when the contract will try to send several requests in one transaction.