Accept forwarding information in transfer tx CLI

[crodriguezvega]

With path forwarding we should allow users of the transfer tx CLI to enter forwarding information.

We can add a new forwarding flag with which users can enter a comma-separated list of hops (e.g transfer/channel-0,transfer/channel-1,transfer/channel-2). The CLI should parse the string into a slice of Hops (during the parsing we can validate that each hop is well formatted, i.e. the port and channel IDs are separated by a slash and both are valid identifiers).

Since we also have memo field in forwarding we could do the following: if a memo string is provided and no forwarding information is provided, then the memo string goes to the regular memo field; if forwarding information is provided, then the memo string goes to the memo field in the forwarding struct.