Create abstraction for oauth client registry

[akondas]

Last part.
Closes #393

[codecov]

Codecov Report

Merging #397 (4ac97e8) into master (14460be) will decrease coverage by 0.01%.
The diff coverage is 97.22%.

@@             Coverage Diff              @@
##             master     #397      +/-   ##
============================================
- Coverage     99.51%   99.50%   -0.02%     
- Complexity     1839     1848       +9     
============================================
  Files           295      297       +2     
  Lines          5804     5823      +19     
============================================
+ Hits           5776     5794      +18     
- Misses           28       29       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/Service/Proxy.php	96.25% <50.00%> (-0.59%)	63.00 <0.00> (+3.00)	⬇️
src/Entity/User/OAuthToken.php	100.00% <100.00%> (ø)	8.00 <5.00> (ø)
...ndler/Organization/Package/AbstractHookHandler.php	100.00% <100.00%> (ø)	1.00 <1.00> (ø)
...r/Organization/Package/AddBitbucketHookHandler.php	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...dler/Organization/Package/AddGitHubHookHandler.php	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...dler/Organization/Package/AddGitLabHookHandler.php	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...rganization/Package/RemoveBitbucketHookHandler.php	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
...r/Organization/Package/RemoveGitHubHookHandler.php	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
...r/Organization/Package/RemoveGitLabHookHandler.php	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
...ackageSynchronizer/ComposerPackageSynchronizer.php	98.23% <100.00%> (ø)	33.00 <0.00> (ø)
... and 5 more

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