Refactor discovery implementation

These changes refactor the discovery implementation a bit to improve
test coverage and remove duplication of common logic shared with the
bundle plugin.

Specifically, the downloading logic has been moved into a separate
package that is shared by bundle and discovery. Second, test coverage in
the discovery implementation is increased from ~15% to ~85%.

These changes also include a few functional improvements:

- The default decision paths can be updated dynamically
- The decision logger can be enabled dynamically
- Discovery downloading errors are reported in status updates
- Discovery bundle is evaluated with all runtime params
- Custom plugins can be created dynamically
- Status updates include both discovery and bundle status

Signed-off-by: Torin Sandall <torinsandall@gmail.com>