Support externalizing attachments

### 🤔 What's the problem you're trying to solve?

Following https://github.com/cucumber/messages/pull/372 we can externalize attachments. This would also allow the messages-cli to externalize attachments.

### ✨ What's your proposed solution?

Add a new command

```
messages-cli externalize-attachments --output-directory= [--replacement-prefix=] [--output=] FILE
```

The replacement prefix will use the FQN of the output directory if not provided.

### ⛏ Have you considered any alternatives or workarounds?

_No response_

### 📚 Any additional context?

_No response_