[ICS20-2] Validate application version and ensure backwards compatibility for sendTransfer

[chatton]

When the Transfer rpc endpoint is hit, in order to maintain backwards compatibility we need to ensure that even though we support ics20-2, it is still possible to have an application version of ics20-1.

To accommodate this we need to ensure that in the case of ics20-1, we only allow a single token in the tokens array.

poc implementation