feat: Video filters

[Brazol]

This PR adds Blur and Image background effects to our Flutter Video SDK.

Docs PR: https://github.com/GetStream/docs-content/pull/50

iOS

ios_filters.mp4

Android

android_filters.mp4

[EvertonMJunior]

Hi! Thanks for this amazing job @Brazol, we are migrating from another provider to GetStream and the experience has been much better. Our usecase requires mainly the blur effects, and we already got it working both on iOS and Android with this PR. We have a due date of mid-January to deliver these changes to our clientes, so I was wondering, is adding video filters to the web implementation something that will be done soon? We have a big usage of our app on the web version, and we want to support blur on the web too, inside our app. We have some more context on the video calls that is already implemented on it.