Support pinning in the Honeycomb.io UI

[bixu]

Having many SLOs, my org likes to pin key SLOs in the UI. Since Terraforming SLOs sometimes requires re-creation, the pins are lost.

Having an attribute for honeycomb_slo for pinning could fix this for us.

[jharley]

Thanks for this suggestion, @bixu!

The API doesn't currently expose this functionality so we'll need to look into that as a starting point.