feat(@xen-orchestra/backups): refactor backup merge code (#9300)

this merge code leverage the new classes introduced to handle qcow2, improving our flexibility, and future uses.

this also improve the performance on "block mode" backups (s3/ azure and other storage with the option), since it reliably updates the block allocation table only at the end of the merge instead of each time a new block is detected

@xen-orchestra/disk-transform/src/FileAccessor.mts

@@ -18,9 +18,12 @@ export interface FileAccessor {
     buffer: Buffer | DataView,
     position: number
   ): Promise<{ bytesRead: number; buffer: Buffer | DataView }>
+  readFile(path: string): Promise<string>
+  writeFile(path: string, content: string, opts?: unknown): Promise<void>
   getSize(path: string): Promise<number>
   mktree(path: string): Promise<void>
   rmtree(path: string): Promise<void>
   unlink(path: string): Promise<void>
   outputStream(stream: Readable): Promise<void>
+  rename(path: string, newPath: string): Promise<void>
 }