fix(ui): Attachment actions on full screen were broken

[ayush221b]

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Description of the pull request

Solves #1071

The FullScreen implementation and the adjoining widgets had some flaws, because of which we were getting some random unexpected behaviour. I have introduced the concept of StreamAttachmentPackage, that can be used when each individual attachment needs to be mapped to its message, I have also added a new progress UI and changed the way we track download completion for attachments. I have also made the necessary changes to the tests.

[ayush221b]

1. This also removes the prefer_expression_function_bodies rule from our analysis options.
2. Also, this exports the file src/multi_overlay.dart because that file houses the class OverlayOptions which we require when a developer has to pass customOverlays to the StreamMessageInput.

[imtoori]

LGTM

[ibhanu]

fyi, it brings breaking changes!

[imtoori]

@ibhanu that's why it's in v4

[ibhanu]

how would it work for backward compatibility?
because old version have this issue.

[imtoori]

it doesn't work :( this was the only way to fix it and we pushed it in v4 since it's breaking
I hope that's not a problem, the upgrade is not hard https://getstream.io/chat/docs/sdk/flutter/guides/migration_guide_4_0/