Feature: Ephemeral action runners (#33570)

* This includes a runner mock test for hardend PickTask behavior like
described in my proposal
* Runner register ephemeral flag introduced in
https://gitea.com/gitea/act_runner/pulls/649

Closes #32461

models/actions/runner.go

@@ -57,6 +57,8 @@ type ActionRunner struct {
 
 	// Store labels defined in state file (default: .runner file) of `act_runner`
 	AgentLabels []string `xorm:"TEXT"`
+	// Store if this is a runner that only ever get one single job assigned
+	Ephemeral bool `xorm:"ephemeral NOT NULL DEFAULT false"`
 
 	Created timeutil.TimeStamp `xorm:"created"`
 	Updated timeutil.TimeStamp `xorm:"updated"`