Allow Substrate Pallet to be Initialized via Dipatchable (#481)

* Add dispatchable for intializing pallet

* Add Polkadot JS types for Substrate bridge pallet

* Ensure Root is the only one that can initialize the pallet

* Add some tests

* Pack initialization data into struct

* Only allow pallet to be initialized once

* Use new initialization config in nodes

* Rename ScheduledChange in Ethereum pallet

We're renaming it to prevent clashes with Substrate bridge pallet type
of the same name. This is relevant when importing types to Polkadot JS
Apps.

* Move all Polkadot JS types into one file

* Appease Clippy

bridges/modules/substrate/src/storage.rs

@@ -24,6 +24,22 @@ use sp_finality_grandpa::{AuthorityList, SetId};
 use sp_runtime::traits::Header as HeaderT;
 use sp_runtime::RuntimeDebug;
 
+/// Data required for initializing the bridge pallet.
+///
+/// The bridge needs to know where to start its sync from, and this provides that initial context.
+#[derive(Default, Encode, Decode, RuntimeDebug, PartialEq, Clone)]
+#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
+pub struct InitializationData<H: HeaderT> {
+	/// The header from which we should start syncing.
+	pub header: H,
+	/// The initial authorities of the pallet.
+	pub authority_list: AuthorityList,
+	/// The ID of the initial authority set.
+	pub set_id: SetId,
+	/// The first scheduled authority set change of the pallet.
+	pub scheduled_change: Option<ScheduledChange<H::Number>>,
+}
+
 /// A Grandpa Authority List and ID.
 #[derive(Default, Encode, Decode, RuntimeDebug, PartialEq, Clone)]
 #[cfg_attr(feature = "std", derive(Serialize, Deserialize))]