Implement URLCache on Android

[mouselangelo]

Type of change: feature

Motivation (current vs expected behavior)

On iOS URLSession APIs automatically provide default In-memory and on-disk caching via the URLCache API.

The URLCache API has not been implemented in swift-corelibs-foundation - the version of Foundation that our Android build chain uses.

This PR implements a basic URLCache that provides Disk caching for Android.

Todos:

• [ ] Remove old response data and files when newer versions of resources become available.
• [ ] Ensure the cache size limits are enforced by removing the oldest resources when needed.

Nice-to-have:

The following are nice to haves and are open for discussion. These can be added later on a need-to basis:

• [ ] More tests!
• [ ] Implement checks for HTTP header Cache-control: max-age... and remove stale caches. Currently only compares Etag
• [ ] Implement StoragePolicy for CachedURLResponse objects.
• [ ] Use SQLite for storing the response data (matching behavior on iOS/macOS)

Not doing:

The following are not part of the scope of this PR.

• In-memory cache, currently not implementing

Please check if the PR fulfills these requirements

• [ ] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [ ] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [ ] The commit messages are clean and understandable
• [ ] Tests for the changes have been added (for bug fixes / features)

[codecov[bot]]

Codecov Report

Merging #288 into master will decrease coverage by 1.26%. The diff coverage is 36.8%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
- Coverage   51.92%   50.66%   -1.26%     
==========================================
  Files          87       91       +4     
  Lines        3168     3456     +288     
==========================================
+ Hits         1645     1751     +106     
- Misses       1523     1705     +182

Impacted Files	Coverage Δ
Sources/TaskRegistry.swift	0% <0%> (ø)
Sources/URLSession+UrlCache.swift	0% <0%> (ø)
Sources/FlowkeyCachedURLResponse.swift	0% <0%> (ø)
Sources/FlowkeyURLCache.swift	74.64% <74.64%> (ø)

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[mouselangelo]

Closing this for now, but keeping the branch