Use a custom user-agent when broadcasting transactions/doing queries

[freak12techno]

Is your feature request related to a problem? Please describe.
I maintain a public node and it's impossible to distinguish dvpn nodes' queries from other ones, because they use the generic "Go http/client" user agent, which does not state what app did that query.

Describe the solution you'd like
When doing queries towards RPC/API nodes, use a custom User-Agent header, maybe something like "sentinel-dvpn-node v0.1.2".

Describe alternatives you've considered
???

Additional context
None so far.