Replace legacy image downloader with new actor-based implementation

[azarovalex]

The current image downloader is based on NSOperationQueue and has some bugs that I previously described in #4587. There is also a chance that these bugs caused memory corruption in some cases.

Instead of supporting sophisticated logic in legacy NSOperation-based implementation, I replaced it with new async/await implementation. It works the same way in fewer lines of code and because it's an actor, the compiler guarantees thread safety.

The only minor downside is that we can't use it on iOS 12, so I added another very simple implementation of the image downloader. It still caches responses but doesn't support reentrancy (meaning that 5 requests one after another will trigger 5 downloads), which is a trade-off we have to make.

TODO:

• add changelog entry

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (ce064ed) 59.79% compared to head (2e0b7fd) 59.43%.

@@            Coverage Diff             @@
##             main    #4588      +/-   ##
==========================================
- Coverage   59.79%   59.43%   -0.36%     
==========================================
  Files         189      189              
  Lines       21218    21037     -181     
==========================================
- Hits        12687    12504     -183     
- Misses       8531     8533       +2     

Files	Coverage Δ
...oxNavigation/ImageDownloader/ImageDownloader.swift	100.00% <100.00%> (ø)
Sources/MapboxNavigation/SpriteRepository.swift	92.59% <100.00%> (-0.21%)	⬇️
...gation/ImageDownloader/LegacyImageDownloader.swift	92.85% <92.85%> (ø)
Sources/MapboxNavigation/ImageRepository.swift	81.81% <72.72%> (-3.29%)	⬇️

... and 1 file with indirect coverage changes

[Udumft]

LGTM with few comments