backup: add is_compacted field to backup manifest

This commit adds an `is_compacted` field to the backup manifest,
allowing us to easily detect when a manifest represents a compacted
backup.

Epic: None

Release note: None

Author: kev-cao

pkg/backup/backuppb/backup.proto

@@ -151,7 +151,9 @@ message BackupManifest {
   int32 elided_prefix = 28 [(gogoproto.nullable) = false,
     (gogoproto.customtype) = "github.com/cockroachdb/cockroach/pkg/sql/execinfrapb.ElidePrefix"];
 
-  // NEXT ID: 29.
+  bool is_compacted = 29;
+
+  // NEXT ID: 30.
 }
 
 message BackupPartitionDescriptor{

pkg/backup/compaction_job.go

@@ -622,6 +622,7 @@ func createCompactionManifest(
 	if err != nil {
 		return nil, err
 	}
+	m.IsCompacted = true
 	m.IntroducedSpans, err = compactIntroducedSpans(ctx, m, compactChain)
 	if err != nil {
 		return nil, err