Add protocol for migration service

Sources/DataRaft/Enums/MigrationError.swift

@@ -0,0 +1,14 @@
+import Foundation
+import DataLiteCore
+
+/// Errors that may occur during migration registration or execution.
+public enum MigrationError<Version: VersionRepresentable>: Error {
+    /// A migration with the same version or script URL was already registered.
+    case duplicateMigration(Migration<Version>)
+    
+    /// Migration execution failed, with optional reference to the failed migration.
+    case migrationFailed(Migration<Version>?, Error)
+    
+    /// The migration script is empty.
+    case emptyMigrationScript(Migration<Version>)
+}