[epic] catalogd formalized web interface

[grokspawn]

The RFC written for #426 identified a desire to formalize the catalogd web API, and divided work into a set of v1.0-blocking changes to enable versioned web interfaces (phase 1) and non-blocking changes to express and extend a formalized API specification (phase 2).

This epic is to track the design and implementation work associated with phase 2. During phase 1 RFC review we identified that we needed more work to capture the extensibility design but didn't want to slow progress on the v1.0 blocking changes so the first step should be an RFC to capture the design goals for phase 2, and then any implementation trackers we feel are necessary.

• catalogd web api phase 2 RFC #452