WIP: Adding multi-valued secrets ("keyrings") to setec

[creachadair]

This is not ready for use yet, but I wanted to put the outlines up for us to talk about. The server API is all implemented and works fine, the integration with the Store type is not yet done.

• types: add GetAllRequest and GetAllResponse
• db: add a getAll method to kv and GetAllFiltered to DB
• server: add /api/get-all
• client/setec: sketch of keyring