Support shield addons

[jaredhirsch]

Requirements aren't firm here, but from discussion on IRC with @groovecoder, containers would like to submit a test pilot-style ping, but with an extra '-shield' tag somewhere.

For legacy bootstrapped extensions, we could just move the test pilot telemetry ping code over to this file.

I'm not sure shield supports pure webextensions yet, but in that case:

• when the addon leaves test pilot, test pilot will stop setting up a BroadcastChannel
• the BroadcastChannel setup code could be moved into a separate module within this library, that's run inside an embedding bootstrap.js
• the telemetry ping code could be packaged up with the broadcastchannel code.

This would effectively shuffle all of the Test Pilot addon metrics code into this library. We could have the Test Pilot addon itself use this library. This might get us one big step closer to deleting the Test Pilot addon altogether. cc @lmorchard and @fzzzy for thoughts here

[jaredhirsch]

It occurs to me that, once the Telemetry API is available to webextensions, we wouldn't need the Test Pilot addon to submit pings at all.