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/migrations/000015_workspacebuild_deadline.down.sql

@@ -0,0 +1 @@
+ALTER TABLE ONLY workspace_builds DROP COLUMN deadline;

coderd/database/migrations/000015_workspacebuild_deadline.up.sql

@@ -0,0 +1 @@
+ALTER TABLE ONLY workspace_builds ADD COLUMN deadline TIMESTAMPTZ NOT NULL DEFAULT TIMESTAMPTZ '0001-01-01 00:00:00+00:00';