Add gghealthd design doc

docs/design/gghealthd-design.md

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+# `gghealthd` design
+
+See [gghealthd spec](../spec/components/gghealthd.md) for the public interface
+for gghealthd.
+
+gghealthd is intended to forward and serve component lifecycle state updates
+between gghealthd's clients and the Greengrass Lite core device's orchestrator.
+Communication with the orchestrator requires its own binary ABI or network
+protocol depending on the system. Therefore, gghealthd is an abstraction over a
+subset orchestration's API. In order to support state updates via Greengrass
+Classic IPC over ggipcd, gghealthd shall be implemented as a core component, a
+service, with permissions to update a component's orchestration state on its
+behalf.
+
+# Responsibilities
+
+![Data flow of gghealthd responsibilities. All communication between other processes is done over core bus, with translation to/from generic components via `ggipcd`](gghealthd.svg)
+
+- On startup, `gghealthd` shall begin serving the `gg_health` Core Bus API
+  interface.
+- When recipe-runner executes, it may send state updates to `gghealthd` for
+  `install` and `run` scripts.
+- When a generic component executes its `start` script, it may send state
+  updates over the IPC bridge.
+- When a deployment starts, gghealthd must receive a subscription request from
+  `ggdeploymentd`. In response, gghealthd shall connect to the core device
+  orchestrator and await Greengrass component completions or failures,
+  forwarding the overall result back to ggdeploymentd.
+- On request by `gg-fleet-statusd`, `gghealthd` shall report individual
+  component lifecycle states. This will be used to report the overall device
+  health as well as what components are installed on the device.
+- `gghealthd` retrieves requested component information from `ggconfigd` for
+  input validation.