Feature: Progress notifications for launch

[nvborisenko]

When agent calls launcher.Sync() it would be nice user sees the progress of sending a report to the server.

var launch = new(...);
launch.CreateManyTests(...); // scheduling a lot of http requests

launch.ProgressCompletion.Changed += (value) => Console.Write($"Progress: {value}%");

launch.Sync(); // waiting all http requests to be sent

Thus user will be able to understand the actual state of sending the report. Actual naming is a subject of discussion. Progress (in percents) should be based on http requests, not on count of tests.

Blocked by: #131

[nvborisenko]

It's also be nice if agent will have a possibility to specify time threshold like "I would like to get notifications not so often, no more often than 1 event per 2 minutes".

It will be strange if agent will output:

1%
2%
...

or even like this, if overall reporting takes 1 second.

10%
20%
...