[refactor] refactor-language-download

[GabrielSousa02]

What?

Create local download manager for languages and remove download_assets package.

• test: create tests for new language_downloader feature
• build: add new archive dependency and bump minor versions
• feat: create new custom language_downloader util
• feat: wire new language_downloader into globals
• feat: wire new language_downloader into background_task
• feat: wire new language_downloader into main
• test: update background_test
• build: update pubspec files to remove download_assets
• refactor: update languages.dart to use the new language_downloader
• refactor: update usage of provider in update_language_button
• refactor: update background_task to use new provider setup
• test: update tests to use new language_downloader and provider
• refactor: remove unnecessary non-null assertion with dart fix

Why?

This removes an unnecessary dependency on the project, which makes it more reliable,
and less prone to update issues. Which was already happening with the latest version
of download_assets.

How?

Created custom download classes and functions that is able to fetch the two files per
language at the same time and unpack the zip in which they come, as per the current
setup of the application and content distribution.

Testing?

New testes were added, and all automated tests passing locally.

Screenshots (optional)

N/A

Anything Else?

N/A

[GabrielSousa02]

@holybiber Here's the refactor I've been working on to remove download_assets package that was being a problem to us.

Tests are passing, and local emulators are working, download actually feels even faster at the moment.

Would like to take a look and test it before I merge it?

[holybiber]

Hi Gabriel, thanks a lot!
This looks good to me. It's good to get rid of download_assets and have our own code instead. I like that you download the pdf and the html archives in parallel, that already doubles the download speed. Did some quick tests on my phone, with the current code downloading all languages took 83s, with this PR it only took 38s.
Can you remove all remaining mentions of download_assets from our documentation?
And then go ahead and merge it!