wait for jobs to complete when stopping the runner

When receiving a signal (INT or TERM) wait for running jobs to
complete instead of terminating them right away.

The new shutdown_timeout configuration setting can be used to force
the termination after a grace delay. If not specified or zero it will
shutdown immediately, for backward compatibility. It will be the case
with existing configuration files or when a configuration file is not
specified.

The config.yml created with the generate-config subcommand will
however default shutdown_timeout to 3h (same as timeout) because it is
likely what a new admin would expect: shutting down waits for jobs to
complete and not abort them.

internal/app/run/runner.go

@@ -41,6 +41,10 @@ type Runner struct {
 	runningTasks sync.Map
 }
 
+type RunnerInterface interface {
+	Run(ctx context.Context, task *runnerv1.Task) error
+}
+
 func NewRunner(cfg *config.Config, reg *config.Registration, cli client.Client) *Runner {
 	ls := labels.Labels{}
 	for _, v := range reg.Labels {