* 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).
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