Add shared ignore list for server and subscriber

[Owen-Cummings]

Allows an ignore list for extensions we don't wish to vouch from the configuration file.

[chrisshino]

lgtm!

[thepwagner]

This LGTM, noting the things we want to verify from our pairing session:

• We assume imageData.String() is the full registry+label (e.g. gcr.io/my-project/my-image:my-tag)
• We assume returning an empty HTTP body won't affect clients, if they get a 200 OK they are happy.

We could consider integrating this with the metrics system, but lets not - the logging signal should be sufficient that we can see it working.