run: print statistics at the end

[fho]

Print a summary with statistics on the end of a baur run execution, some ideas what could be shown:

• a table with: - task name with status - output URLs and sizes - build an
• total runtime
• number of run task, number of successful tasks
• number of uploaded files, number of successful uploads
• task that took the longest time to execute
• artifact that took the longest time to upload

Similar idea then: https://github.com/simplesurance/baur/issues/151