Add a command to send notifications

[vicero]

In a monorepo with multiple test suites, it would be nice to add a command to the codecov orb that sends notifications. Otherwise I have to reproduce all the steps of the orb in order to send the notifications myself.

[thomasrockhu-codecov]

@vicero this has been done with the argument run_command, you should be able to set run_command: "send-notifications"