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/database/models.go

@@ -505,6 +505,7 @@ type WorkspaceBuild struct {
 	InitiatorID       uuid.UUID           `db:"initiator_id" json:"initiator_id"`
 	ProvisionerState  []byte              `db:"provisioner_state" json:"provisioner_state"`
 	JobID             uuid.UUID           `db:"job_id" json:"job_id"`
+	Deadline          time.Time           `db:"deadline" json:"deadline"`
 }
 
 type WorkspaceResource struct {