Add support for ImageStreamListener.onChunk()

[tvolkert]

Description

This is another step towards supporting image loading
progress notification at the widgets layer.

This adds an ImageChunkEvent class along with associated
ImageChunkListener callback signature and an onChunk
property to ImageStreamListener. The events serve to
notify registered listeners when byte chunks are received
while loading an image.

Related Issues

#32374

Tests

I added the following tests:

• Verification that an ImageProvider notifies listeners of chunk events
• Verification that MultiFrameImageStreamCompleter delivers chunk events to listeners
• Verification that MultiFrameImageStreamCompleter drops chunk events on the floor before listeners are registered (intended behavior).
• Verification that errors reported on the chunk stream that's passed to MultiFrameImageStreamCompleter are properly reported.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[goderbauer]

LGTM