[doc] Example MBM pallet (#2119)

## Basic example showcasing a migration using the MBM framework

This PR has been built on top of
https://github.com/paritytech/polkadot-sdk/pull/1781 and adds two new
example crates to the `examples` pallet

### Changes Made:

Added the `pallet-example-mbm` crate: This crate provides a minimal
example of a pallet that uses MBM. It showcases a storage migration
where values are migrated from a `u32` to a `u64`.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/examples/multi-block-migrations/src/mock.rs

@@ -0,0 +1,93 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#![cfg(test)]
+
+//! # Mock runtime for testing Multi-Block-Migrations
+//!
+//! This runtime is for testing only and should *never* be used in production. Please see the
+//! comments on the specific config items. The core part of this runtime is the
+//! [`pallet_migrations::Config`] implementation, where you define the migrations you want to run
+//! using the [`Migrations`] type.
+
+use frame_support::{
+	construct_runtime, derive_impl,
+	migrations::MultiStepMigrator,
+	pallet_prelude::Weight,
+	traits::{OnFinalize, OnInitialize},
+};
+
+type Block = frame_system::mocking::MockBlock<Runtime>;
+
+impl crate::Config for Runtime {}
+
+frame_support::parameter_types! {
+	pub storage MigratorServiceWeight: Weight = Weight::from_parts(100, 100); // do not use in prod
+}
+
+#[derive_impl(pallet_migrations::config_preludes::TestDefaultConfig)]
+impl pallet_migrations::Config for Runtime {
+	// Here we inject the actual MBMs. Currently there is just one, but it accepts a tuple.
+	//
+	// # Example
+	// ```ignore
+	// type Migrations = (v1::Migration<Runtime>, v2::Migration<Runtime>, v3::Migration<Runtime>);
+	// ```
+	#[cfg(not(feature = "runtime-benchmarks"))]
+	type Migrations = (
+		crate::migrations::v1::LazyMigrationV1<
+			Runtime,
+			crate::migrations::v1::weights::SubstrateWeight<Runtime>,
+		>,
+	);
+	#[cfg(feature = "runtime-benchmarks")]
+	type Migrations = pallet_migrations::mock_helpers::MockedMigrations;
+	type MaxServiceWeight = MigratorServiceWeight;
+}
+
+#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+impl frame_system::Config for Runtime {
+	type Block = Block;
+	type MultiBlockMigrator = Migrator;
+}
+
+// Construct the runtime using the `construct_runtime` macro, specifying the pallet_migrations.
+construct_runtime! {
+	pub struct Runtime
+	{
+		System: frame_system,
+		Pallet: crate,
+		Migrator: pallet_migrations,
+	}
+}
+
+pub fn new_test_ext() -> sp_io::TestExternalities {
+	sp_io::TestExternalities::new(Default::default())
+}
+
+#[allow(dead_code)]
+pub fn run_to_block(n: u64) {
+	assert!(System::block_number() < n);
+	while System::block_number() < n {
+		let b = System::block_number();
+		AllPalletsWithSystem::on_finalize(b);
+		// Done by Executive:
+		<Runtime as frame_system::Config>::MultiBlockMigrator::step();
+		System::set_block_number(b + 1);
+		AllPalletsWithSystem::on_initialize(b + 1);
+	}
+}