Introduce CollectCollationInfo runtime api (#443)

* Introduce `CollectCollationInfo` runtime api

Instead of using well known keys to communicate information about a
collation between the runtime and the collator, we now use a runtime api
for this.

* Fixes bug

* Apply suggestions from code review

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Doc update

Co-authored-by: Sergei Shulepov <sergei@parity.io>

cumulus/Cargo.lock

@@ -1283,6 +1283,7 @@ dependencies = [
  "polkadot-overseer",
  "polkadot-primitives",
  "sc-client-api",
+ "sp-api",
  "sp-blockchain",
  "sp-consensus",
  "sp-core",
@@ -1565,6 +1566,7 @@ dependencies = [
  "polkadot-core-primitives",
  "polkadot-parachain",
  "polkadot-primitives",
+ "sp-api",
  "sp-runtime",
  "sp-std",
  "sp-trie",