more clear randomness API for BABE (#8180)

* more clear randomness API for BABE

* babe: move randomness utilities to its own file

* node: use babe::RandomnessFromOneEpochAgo in random_seed implementation

* frame-support: annotate randomness trait with block number

* pallet-randomness-collective-flip: fix for new randomness trait

* pallet-society: fix randomness usage

* pallet-lottery: fix randomness usage

* pallet-contracts: fix randomness usage

* pallet-babe: fix randomness usage

we need to track when the current and previous epoch started so that we
know the block number by each existing on-chain was known

* node: fix random_seed

* node-template: fix random_seed

* frame-support: extend docs

* babe: add test for epoch starting block number tracking

* babe: fix epoch randomness docs

* frame: add todos for dealing with randomness api changes

Co-authored-by: André Silva <andrerfosilva@gmail.com>

substrate/Cargo.lock

@@ -1814,7 +1814,7 @@ dependencies = [
 
 [[package]]
 name = "frame-support-test"
-version = "2.0.1"
+version = "3.0.0"
 dependencies = [
  "frame-metadata",
  "frame-support",
@@ -4983,6 +4983,7 @@ version = "3.0.0"
 dependencies = [
  "frame-benchmarking",
  "frame-support",
+ "frame-support-test",
  "frame-system",
  "pallet-balances",
  "parity-scale-codec",
@@ -5278,6 +5279,7 @@ name = "pallet-society"
 version = "3.0.0"
 dependencies = [
  "frame-support",
+ "frame-support-test",
  "frame-system",
  "pallet-balances",
  "parity-scale-codec",