Add partial data handling (ie. Progressive JPEG)

[glemartret]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

This PR Introduces Progressive JPEG loading and prepare the handling of other "progressive" formats

⤵️ What is the current behavior?

This package don't provide partial data during the download, therefore it's impossible to use partial data for compatible files

🆕 What is the new behavior (if this is a feature change)?

I added a new FileResponse type that extends DownloadProgress but contains usable partial data in it

💥 Does this PR introduce a breaking change?

None

🐛 Recommendations for testing

📝 Links to relevant issues/docs

Issue #43 in flutter_cached_network_image

🤔 Checklist before submitting

• Check memory efficiency
• Write tests for the feature
• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current

PS: This is my first try at publicly contributing to a repo, any feedback would be appreciated