feat(API): Add API server code for GET /machines
feat(API): Socket-activate API server
feat(API): Establish client.API interface
feat(API): Add client.HTTPClient
feat(API): Allow user of fleetctl to use HTTPClient

The --experimental-api option instructs fleetctl to use the new
HTTPClient over /var/run/fleet.sock. This option is only available
while the v1 API is under active development.