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/dump.sql

@@ -291,7 +291,8 @@ CREATE TABLE workspace_builds (
     transition workspace_transition NOT NULL,
     initiator_id uuid NOT NULL,
     provisioner_state bytea,
-    job_id uuid NOT NULL
+    job_id uuid NOT NULL,
+    deadline timestamp with time zone DEFAULT '0001-01-01 00:00:00+00'::timestamp with time zone NOT NULL
 );
 
 CREATE TABLE workspace_resources (