feat: add an endpoint to manually resume a coder task (#21948)

Closes https://github.com/coder/internal/issues/1262.

This PR adds:
* the `POST /api/experimental/tasks/{user}/{task}/resume` endpoint
* follows conventions from https://github.com/coder/internal/issues/1261
* sets the build reason to `task_resume`
* a task that is not paused (ie. is already running), cannot be resumed.

coderd/coderd.go

@@ -1079,6 +1079,7 @@ func New(options *Options) *API {
 					r.Post("/send", api.taskSend)
 					r.Get("/logs", api.taskLogs)
 					r.Post("/pause", api.pauseTask)
+					r.Post("/resume", api.resumeTask)
 				})
 			})
 		})