Support remote zilla configuration with change detection

[jfallows]

Describe the desired outcome from the user's perspective
As devops, I want to deploy zilla with zilla.yaml and referenced resources served remotely with change detection.

Acceptance criteria

• Support retrieval of zilla.yaml and relative resources via http(s) with relative paths
• Support long polling via prefer: wait=N and if-none-match: <etag> to detect change
• Support (sparse) polling when prefer: wait=N is not supported

Additional context
Change detection may currently require a new identical zilla.yaml to trigger awareness of other resource change.