Support for remote content repository where all contents and
manifests are fetched over HTTP(S) instead of locally
manipulating blob storage
* server: implement content and manifest access APIs
* apiclient: moved Kopia API client to separate package
* content: exposed content.ValidatePrefix()
* manifest: added JSON serialization attributes to EntryMetadata
* repo: changed repo.Open() to return Repository instead of *DirectRepository
* repo: added apiServerRepository
* cli: added 'kopia repository connect server'
This sets up repository connection via the API server instead of
directly-manipulated storage.
* server: add support for specifying a list of usernames/password via --htpasswd-file
* tests: added API server repository E2E test
* server: only return manifests (policies and snapshots) belonging to authenticated user
/api/v1/repo/create
/api/v1/repo/connect
/api/v1/repo/disconnect
Refactored server code and fixed a number of outstanding robustness
issues. Tweaked the API responses a bit to make more sense when consumed
by the UI.
