Chain API subsystem (#1498)

* chain-api subsystem skeleton

* chain-api subsystem: draft impl

* chain-api subsystem: mock testclient

* chain-api subsystem: impl HeaderBacked for TestClient

* chain-api subsystem: impl basic tests

* chain-api subsystem: tiny guide

* chain-api subsystem: rename ChainApiRequestMessage to ChainApiMessage

* chain-api subsystem: add the page to the ToC

* chain-api subsystem: proper error type

* chain-api subsystem: impl ancestors request

* chain-api subsystem: tests for ancestors request

* guide: fix ancestor return type

* runtime-api subsystem: remove unused dep

* fix fmt

* fix outdated comment

* chain-api subsystem: s/format/to_string

* lower-case subsystem names

* chain-api subsystem: resolve Finalized todo

* chain-api subsystem: remove TODO

* extract request errors into a module

* remove caching TODO

* fix imports

polkadot/roadmap/implementers-guide/src/node/utility/chain-api.md

@@ -0,0 +1,19 @@
+# Chain API
+
+The Chain API subsystem is responsible for providing a single point of access to chain state data via a set of pre-determined queries.
+
+## Protocol
+
+Input: [`ChainApiMessage`](../../types/overseer-protocol.md#chain-api-message)
+
+Output: None
+
+## Functionality
+
+On receipt of `ChainApiMessage`, answer the request and provide the response to the side-channel embedded within the request.
+
+Currently, the following requests are supported:
+* Block hash to number
+* Finalized block number to hash
+* Last finalized block number
+* Ancestors