feat: support callback in retryDelay

[escral]

🔗 Linked issue

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Calculate retry delay based on context. Opens possibility to make retries with incremental delay. For example:

{
  retry: 3,
  retryDelay: (context) => {
      const attempt = context.options.retryAttempt = (context.options.retryAttempt || 0) + 1;
      return 300 * (Math.pow(3, attempt - 1))
  },
}

📝 Checklist

• [ ] I have linked an issue or discussion.
• [ ] I have updated the documentation accordingly.

[escral]

Though I don't like having this context.options "hack". Would be nice to have shared context between retries.