Add download progress bar

[anhnmt]

I've added a feature that I find necessary, which is the progressbar. I'm having a bit of trouble: https://github.com/gotd/td/pull/1430/files#diff-542426cfaa760b18e8d3570ce615670687cc1937b86140c4af5b67f419088a6cR34-R37

I hope someone will help me optimize this code :)

[codecov]

Codecov Report

Attention: Patch coverage is 2.50000% with 39 lines in your changes missing coverage. Please review.

Project coverage is 70.98%. Comparing base (ac6bc17) to head (13e2d89).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
telegram/downloader/parallel.go	4.16%	21 Missing and 2 partials ⚠️
syncio/writer_at.go	0.00%	10 Missing ⚠️
telegram/downloader/downloader.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1430      +/-   ##
==========================================
- Coverage   71.02%   70.98%   -0.04%     
==========================================
  Files         442      442              
  Lines       17744    17783      +39     
==========================================
+ Hits        12602    12624      +22     
- Misses       4209     4220      +11     
- Partials      933      939       +6     

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[ernado]

I'm not sure that this should be in main repo, probably https://github.com/gotd/contrib suits better.

Current repo is for generic things.

Also, https://github.com/iyear/tdl managed to implement a progress bar, so it should be possible.