1. use [staticcheck](https://staticcheck.io/) to check the code, and fix some issues.
2. use `go fmt` to format the code.
3. use `go mod tidy` clean the go mod.
- Add endpoints for osquery to register and continue a carve.
- Implement client functionality for retrieving carve details and contents in fleetctl.
- Add documentation on using file carving with Fleet.
Addresses kolide/fleet#1714
