config: Add parquetProcessingMode to Savepoints configuration

Introduce ParquetProcessingMode enum with two variants:
- Parallel: Process all Parquet files at once (default, no savepoints)
- Sequential: Process files one by one with savepoint support

Add parquetProcessingMode field to Savepoints configuration with
appropriate Circe encoders/decoders. The mode defaults to Parallel
to maintain backward compatibility with existing configurations.

This lays the foundation for implementing savepoint support for
Parquet migrations, allowing users to resume interrupted migrations
without reprocessing already-migrated files.

Author: Artem

migrator/src/main/scala/com/scylladb/migrator/config/Savepoints.scala

@@ -3,7 +3,37 @@ package com.scylladb.migrator.config
 import io.circe.{ Decoder, Encoder }
 import io.circe.generic.semiauto.{ deriveDecoder, deriveEncoder }
 
-case class Savepoints(intervalSeconds: Int, path: String)
+sealed trait ParquetProcessingMode
+object ParquetProcessingMode {
+  case object Parallel extends ParquetProcessingMode
+  case object Sequential extends ParquetProcessingMode
+
+  implicit val encoder: Encoder[ParquetProcessingMode] = Encoder.encodeString.contramap {
+    case Parallel   => "parallel"
+    case Sequential => "sequential"
+  }
+
+  implicit val decoder: Decoder[ParquetProcessingMode] = Decoder.decodeString.emap {
+    case "parallel"   => Right(Parallel)
+    case "sequential" => Right(Sequential)
+    case other =>
+      Left(s"Unknown parquet processing mode: $other. Valid values: parallel, sequential")
+  }
+}
+
+case class Savepoints(intervalSeconds: Int,
+                      path: String,
+                      parquetProcessingMode: Option[ParquetProcessingMode]) {
+
+  /**
+    * Returns the configured Parquet processing mode.
+    * Defaults to [[ParquetProcessingMode.Parallel]] if not specified.
+    * This default affects migration semantics.
+    */
+  def getParquetProcessingMode: ParquetProcessingMode =
+    parquetProcessingMode.getOrElse(ParquetProcessingMode.Parallel)
+}
+
 object Savepoints {
   implicit val encoder: Encoder[Savepoints] = deriveEncoder[Savepoints]
   implicit val decoder: Decoder[Savepoints] = deriveDecoder[Savepoints]