feat: send exn messages from command line

[lenkan]

Implements a kli exn send command that accepts

• --data for the a field.
• --topic for forwarded messages
• --route the route for the exn message
• --sender and --recipient

This can later be expanded to handle --embeds as well. I have a draft for kli exn query, but it is a bit unclear how that is expected to work, for example when there are multiple messages matching the filter etc.

This command can handle challenge responses (see added test cases), but also be used to send generic exn messages to recipients that know how to handle them. Saves us having to implement a new kli command to try out a new route.

I am using it in a proof of concept I am doing, so I thought I'd at least offer to contribute it.