Working sets: resolve server schemas/snapshots and cache them

[cmrigney]

What I did
In preparation for getting working sets fully running in the gateway, this resolves the schema for running the server (as catalog.Server for now since the gateway knows how to run that).

• The resolved schema (aka snapshot) is cached in the DB with the server
• We include the snapshot when we docker mcp workingset show my-set --format json/yaml but remove it on export and pull (the thought was that the file should be more user-editable and including the snapshot would make it fragile)
• Server snapshots are resolved when a working set is either imported from a file, pulled, or first created.

A couple other notes:

• Gap: The registry snapshot is not implemented yet, just the self-describing image snapshot. I'll revisit this.
• Breaking: All images used must be self-describing. We can debate if we want to allow non-self-describing images (in the case when no config is needed). Thoughts @bobbyhouse ?

Related issue

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[bobbyhouse]

Nice work!

I agree on going with all images must be self-describing for workingsets. We may want to add a function to make creating that metadata easy in the future.