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".

backend/compress/unknown_handler.go

@@ -0,0 +1,65 @@
+package compress
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/chunkedreader"
+)
+
+// unknownModeHandler implements compressionModeHandler for unknown compression types
+type unknownModeHandler struct{}
+
+// isCompressible checks the compression ratio of the provided data and returns true if the ratio exceeds
+// the configured threshold
+func (unk *unknownModeHandler) isCompressible(r io.Reader, compressionMode int) (bool, error) {
+	return false, fmt.Errorf("unknown compression mode %d", compressionMode)
+}
+
+// newObjectGetOriginalSize returns the original file size from the metadata
+func (unk *unknownModeHandler) newObjectGetOriginalSize(meta *ObjectMetadata) (int64, error) {
+	return 0, nil
+}
+
+// openGetReadCloser opens a compressed object and returns a ReadCloser in the Open method
+func (unk *unknownModeHandler) openGetReadCloser(
+	ctx context.Context,
+	o *Object,
+	offset int64,
+	limit int64,
+	cr chunkedreader.ChunkedReader,
+	closer io.Closer,
+	options ...fs.OpenOption,
+) (rc io.ReadCloser, err error) {
+	return nil, fmt.Errorf("unknown compression mode %d", o.meta.Mode)
+}
+
+// processFileNameGetFileExtension returns the file extension for the given compression mode
+func (unk *unknownModeHandler) processFileNameGetFileExtension(compressionMode int) string {
+	return ""
+}
+
+// putCompress compresses the input data and uploads it to the remote, returning the new object and its metadata
+func (unk *unknownModeHandler) putCompress(
+	ctx context.Context,
+	f *Fs,
+	in io.Reader,
+	src fs.ObjectInfo,
+	options []fs.OpenOption,
+	mimeType string,
+) (fs.Object, *ObjectMetadata, error) {
+	return nil, nil, fmt.Errorf("unknown compression mode %d", f.mode)
+}
+
+// putUncompressGetNewMetadata returns metadata in the putUncompress method for a specific compression algorithm
+func (unk *unknownModeHandler) putUncompressGetNewMetadata(o fs.Object, mode int, md5 string, mimeType string, sum []byte) (fs.Object, *ObjectMetadata, error) {
+	return nil, nil, fmt.Errorf("unknown compression mode")
+}
+
+// This function generates a metadata object for sgzip.GzipMetadata or SzstdMetadata.
+// Warning: This function panics if cmeta is not of the expected type.
+func (unk *unknownModeHandler) newMetadata(size int64, mode int, cmeta any, md5 string, mimeType string) *ObjectMetadata {
+	return nil
+}