Block reward contract (#8419)

* engine: add block reward contract abi and helper client

* aura: add support for block reward contract

* engine: test block reward contract client

* aura: test block reward contract

* engine + aura: add missing docs

* engine: share SystemCall type alias

* aura: add transition for block reward contract

* engine: fix example block reward contract source link and bytecode

json/src/spec/authority_round.rs

@@ -16,6 +16,7 @@
 
 //! Authority params deserialization.
 
+use ethereum_types::Address;
 use uint::Uint;
 use super::ValidatorSet;
 
@@ -43,6 +44,13 @@ pub struct AuthorityRoundParams {
 	/// Reward per block in wei.
 	#[serde(rename="blockReward")]
 	pub block_reward: Option<Uint>,
+	/// Block at which the block reward contract should start being used.
+	#[serde(rename="blockRewardContractTransition")]
+	pub block_reward_contract_transition: Option<Uint>,
+	/// Block reward contract address (setting the block reward contract
+	/// overrides the static block reward definition).
+	#[serde(rename="blockRewardContractAddress")]
+	pub block_reward_contract_address: Option<Address>,
 	/// Block at which maximum uncle count should be considered.
 	#[serde(rename="maximumUncleCountTransition")]
 	pub maximum_uncle_count_transition: Option<Uint>,