Add multi-block migration to clear storage items under a specific storage prefix

[RomarQ]

Overview

This PR introduces a new generic migration utility ClearStorageByPrefix that enables safe removal of storage items by prefix using the multi-block migration frame.

Motivation

Previously, clearing specific storage items required writing custom migrations for each use case. This generic implementation provides a reusable solution that can clear any storage prefix without being tied to a specific pallet, making storage cleanup operations more maintainable and less error-prone.

Changes

New Migration Utility

• ClearStorageByPrefix<T, StoragePrefix>: A generic struct that implements SteppedMigration to safely clear storage items matching a given prefix
  • Uses the multi-block migration pallet to safely handle large storage removals
  • Processes keys in batches based on available block weight
  • Generates unique migration identifiers based on the storage prefix

Usage Example

use frame_support::parameter_types;

// Define the storage prefix to clear
parameter_types! {
    pub StoragePrefixToClear: [u8; 32] = frame_support::storage::storage_prefix(
        b"PalletName",
        b"StorageName",
    );
}

// Configure the migration
pub type MultiBlockMigrations =
    pallet_migrations::ClearStorageByPrefix<Runtime, StoragePrefixToClear>;

impl pallet_migrations::Config for Runtime {
    type Migrations = MultiBlockMigrations;
    // ... other configuration
}

Breaking Changes

None. This is a new utility that does not affect existing functionality.