feat(server): migrate extensible enum rule to context object to support openapi 3

[maxim-tschumak]

Related to #714

[codecov-io]

Codecov Report

Merging #810 into master will increase coverage by 0.29%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##             master     #810      +/-   ##
============================================
+ Coverage     84.35%   84.65%   +0.29%     
+ Complexity      957      928      -29     
============================================
  Files           171      171              
  Lines          2647     2627      -20     
  Branches        407      397      -10     
============================================
- Hits           2233     2224       -9     
  Misses          198      198              
+ Partials        216      205      -11

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/de/zalando/zally/util/OpenApiUtil.kt	59.57% <100%> (+10.92%)	0 <0> (ø)	⬇️
...e/zalando/zally/rule/zalando/ExtensibleEnumRule.kt	80% <75%> (+14.28%)	6 <5> (-29)	⬇️

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[tkrop]

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[maxim-tschumak]

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