Add ability to "ping" a receiver to help with discovery

[scottlow]

In Berlin, we discussed the ability to "ping" a display to help users disambiguate between multiple receivers (perhaps with similar names) that may show up in a list of named receivers. This would be useful in a meeting room with multiple displays, for example.

This is not currently not possible given the way the protocol is specified today, so we agreed to track this as a v2 issue. The user workflow in the time being will be to leverage the authentication UI (such as the receiver showing a pin) to verify that the receiver chosen was the correct one.