Image cache with resize functionality

[renefloor]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Feature

:arrow_heading_down: What is the current behavior?

Currently you can't cache resized images

:new: What is the new behavior (if this is a feature change)?

When asking for a specific size it checks the cache. If it is not in the cache it downloads and caches the original and caches the resized version as well. It uses the new key logic for that, for example resized_h100_w200_url.

It is implemented as a mixin on the BaseCacheManager.

:boom: Does this PR introduce a breaking change?

No

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

:thinking: Checklist before submitting

• [X] All projects build
• [X] Follows style guide lines (code style guide)
• [x] Relevant documentation was updated
• [x] Rebased onto current develop

[codecov[bot]]

Codecov Report

Merging #236 (d9061bd) into develop (b3abecd) will increase coverage by 1.46%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #236      +/-   ##
===========================================
+ Coverage    74.64%   76.11%   +1.46%     
===========================================
  Files           20       21       +1     
  Lines          635      674      +39     
===========================================
+ Hits           474      513      +39     
  Misses         161      161              

Impacted Files	Coverage Δ
.../lib/src/cache_managers/default_cache_manager.dart	0.00% <ø> (ø)
...er/lib/src/cache_managers/image_cache_manager.dart	100.00% <100.00%> (ø)

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