feat: add API/SDK support for autostop extension (#1778)

* Adds deadline column to workspace_builds, associated DB/API plumbing
* database: Upon inserting a row into workspace_builds, deadline will 
  initially be zero.
* autobuild: Executor now checks the Deadline field of the workspace_build
  for the purpose of autostop logic.
* coderd: Adds a new route /api/v2/workspaces/:workspace/extend which allows
  updating the deadline of the currently active workspace build. The new
  deadline must be after the existing deadline, and not the zero time.
* provisionerd: updates workspace_build.deadline upon successful workspace 
  build completion (equal to now plus workspace TTL, if it exists).

coderd/coderd.go

@@ -315,6 +315,7 @@ func New(options *Options) *API {
 					r.Put("/", api.putWorkspaceTTL)
 				})
 				r.Get("/watch", api.watchWorkspace)
+				r.Put("/extend", api.putExtendWorkspace)
 			})
 		})
 		r.Route("/workspacebuilds/{workspacebuild}", func(r chi.Router) {