The main motivation behind this change is to get just a little
telemetry information from nodes on the network, namely the version
of Radicle they are running.
This is achieved by rewriting `impl Default for UserAgent` which now
uses the version information provided by the build script also used
in other crates.
Also, a new configuration option `node.userAgent` is added, which
allows users to override the user agent if they so please, or set
the value `null`, which will in turn send the user agent
`/radicle/`, which is not really helpful, and the default prior to
this commit.
Creations of `UserAgent` in the whole workspace is cleaned up. In
order to do that `UserAgent::test` is introduced.
Lorenz Leutgeb