Obnam server API: GET and PUT chunks

Goal The goal for today is to implement API endpoints to upload and download chunks. Plan Add an endpoint for PUT /chunks/ID?label=LABEL . Add an endpoint for GET /chunks/ID Add verification scenario or scenarios for the endpoints. Notes Preparation: existing code builds and tests pass. in a checkout of main , run make it succeeds all good Create branch chunks . In obnam-server/src/api.rs add a…