Aura and Slots refactoring (#8386)

* Make slot duration being exposed as `Duration` to the outside

* Some slot info love

* Add `build_aura_worker` utility function

* Copy copy copy

substrate/Cargo.lock

@@ -7208,6 +7208,7 @@ dependencies = [
  "sp-inherents",
  "sp-runtime",
  "sp-state-machine",
+ "sp-timestamp",
  "sp-trie",
  "substrate-test-runtime-client",
  "thiserror",
@@ -8559,6 +8560,7 @@ dependencies = [
  "parity-scale-codec",
  "sp-api",
  "sp-application-crypto",
+ "sp-consensus",
  "sp-consensus-slots",
  "sp-inherents",
  "sp-runtime",