This commit aims to introduce a simple API for end users.

Working with network especially Tor routers is flaky and
delegating retry logic implementation to users causes NOnion to
be unreliable in CI and in normal use, this commit introduces
retry logic in some places where it's needed the most. This
should hopefully make NOnion more reliable.

Client,Network,Services: add TorClient

This commit upgrades test project and our CI to use .NET 6.
There's no excuse to use .netcoreapp 3.1 anymore.

It's noteworthy to say that we will keep NOnion target platform
on netstandard2.0 for the time being.

P.S:
My reason for this upgrade was mostly lack of native apple
silicon support in .NET Core 3.1.