Add partial data handling (ie. Progressive JPEG)

[glemartret]

:sparkles: 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

:arrow_heading_down: 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

:new: 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

:boom: Does this PR introduce a breaking change?

No

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

Issue #43 in flutter_cached_network_image

:thinking: Checklist before submitting

• [ ] Check memory efficiency
• [ ] 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