Add `Progressing` condition type?

[joelanford]

As of #217, we have removed the Ready condition, and we added the Installed condition in #218 and Resolved condition in #213.

We may want to consider another condition type called Progressing that tells a user that the controller is working toward (or not) achieving the desired state specified in the operator spec.

This would be useful to understand context when status.installedBundleSource and status.resolvedBundleSource differ.