Support additional keys from non-interactive LaunchAgent

[call]

Buildkite provides two example LaunchAgent plists:

• https://github.com/buildkite/agent/blob/master/templates/launchd.plist
• https://github.com/buildkite/agent/blob/master/templates/launchd_local_with_gui.plist

The current buildkite_agent::service implementation is based on the launchd_local_with_gui.plist. The buildkite_agent::service defined type should be updated to support configuration keys from both example plists.