Add a custom User-Agent to rly queries

[freak12techno]

When rly does its queries to the node, it doesn't override the User-Agent header, so instead, a default Golang User-Agent is used, and that makes it impossible to distinguish the queries done by rly and by any other tools written in Golang that doesn't have the User-Agent overridden.

This, in turn, makes it impossible to see the queries done by rly when looking at the logs of the queries done towards the node.

I suggest customizing the User-Agent header to have a value like rly/x.y.x, so it would be possible, and so if you look at the logs, you see which rly version did these logs.