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* fix outdated comment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/SUMMARY.md

@@ -39,6 +39,7 @@
     - [Misbehavior Arbitration](node/utility/misbehavior-arbitration.md)
     - [Peer Set Manager](node/utility/peer-set-manager.md)
     - [Runtime API Requests](node/utility/runtime-api.md)
+    - [Chain API Requests](node/utility/chain-api.md)
 - [Data Structures and Types](types/README.md)
     - [Candidate](types/candidate.md)
     - [Backing](types/backing.md)