Add .zst extension for image2disk (#160)

## Description

This PR adds support for .zst as file extension for zstd-compressed images, since .zst is the extension recommended in the zstandard RFC.

It also updates to Readme to properly mention the supported file extensions

Fixes: #

## How Has This Been Tested?

A working example of ours using zstd-compressed images with .zs extension has been modified to use .zst

## How are existing users impacted? What migration steps/scripts do we need?

The `.zs` extension is kept for backwards compatibility, therefore existing users are not impacted

## Checklist:

I have:

- [x] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

Author: mergify[bot]

image2disk/README.md

@@ -58,7 +58,7 @@ actions:
 
 ## Supported Compression formats
 
-- bzip2 (`.bzip2`)
+- bzip2 (`.bzip2`, `.bz2`)
 - gzip (`.gz`)
 - xz (`.xz`)
-- xs (`.xs`)
+- zstd (`.zs`, `.zst`)

image2disk/image/image.go

@@ -186,7 +186,7 @@ func findDecompressor(imageURL string, r io.Reader) (io.ReadCloser, error) {
 			return nil, fmt.Errorf("[ERROR] New xz reader: %w", err)
 		}
 		return io.NopCloser(reader), nil
-	case ".zs":
+	case ".zs", ".zst":
 		reader, err := zstd.NewReader(r)
 		if err != nil {
 			return nil, fmt.Errorf("[ERROR] New zs reader: %w", err)