backend/compress: add zstd compression

Added support for reading and writing zstd-compressed archives in seekable format
using "github.com/klauspost/compress/zstd" and
"github.com/SaveTheRbtz/zstd-seekable-format-go/pkg".

Bumped Go version from 1.24.0 to 1.24.4 due to requirements of
"github.com/SaveTheRbtz/zstd-seekable-format-go/pkg".

fstest/test_all/config.yaml

@@ -133,6 +133,9 @@ backends:
  - backend:  "compress"
    remote:   "TestCompress:"
    fastlist: false
+ - backend:  "compress"
+   remote:   "TestCompressZstd:"
+   fastlist: false
  # - backend:  "compress"
  #   remote:   "TestCompressSwift:"
  #   fastlist: false