Add basic support for Composer v2

[karniv00l]

As requested in #109

• proxy support for metadata-url (I've used some of the code Composer V2 Support #100, thanks to @sadortun)
• repo support for metadata-url

[codecov]

Codecov Report

Merging #205 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #205   +/-   ##
=========================================
  Coverage     99.82%   99.82%           
- Complexity     1404     1412    +8     
=========================================
  Files           231      231           
  Lines          3973     4001   +28     
=========================================
+ Hits           3966     3994   +28     
  Misses            7        7           

Impacted Files	Coverage Δ	Complexity Δ
src/Controller/ProxyController.php	100.00% <100.00%> (ø)	9.00 <1.00> (+1.00)
src/Controller/RepoController.php	100.00% <100.00%> (ø)	15.00 <2.00> (+2.00)
src/Service/Proxy.php	100.00% <100.00%> (ø)	30.00 <5.00> (+5.00)
...oxy/MetadataProvider/CacheableMetadataProvider.php	100.00% <100.00%> (ø)	6.00 <0.00> (ø)

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