feat: add an endpoint to manually pause a coder task (#21889)

Closes https://github.com/coder/internal/issues/1261.

This pull request adds an endpoint to pause coder tasks by stopping the
underlying workspace.
* Instead of `POST /api/v2/tasks/{user}/{task}/pause`, the endpoint is
currently experimental.
* We do not currently set the build reason to `task_manual_pause`,
because build reasons are currently only used on stop transitions.

coderd/coderd.go

@@ -1078,6 +1078,7 @@ func New(options *Options) *API {
 					r.Patch("/input", api.taskUpdateInput)
 					r.Post("/send", api.taskSend)
 					r.Get("/logs", api.taskLogs)
+					r.Post("/pause", api.pauseTask)
 				})
 			})
 		})