Add external property to coder_app

feat: Add support for shutdown_script (coder#79)

feat: Add motd_file to coder_agent (coder#78)

This change allows us to enable better feature parity with OpenSSH by
being able to display a message when users login. This is useful for
template authors who want to inform their users about changes or
reporting about the state of the workspace.

Rename `cost` to `daily_cost` (coder#75)

Add cost field to resource_metadata (coder#74)

feat: Add connection_timeout and troubleshooting_url to agent (coder#71)

Related: coder/coder#4678