[WIP] Add rules for nullable cases

[macisamuele]

This PR fixes #22 by adding two new rules: RES-E004 and REQ-E004.

Currently the rules are fully implemented and tested.

The PR is labeled as WIP as documentation is still missing.

Side note: The skeletons (first two commits of this PR) were created with #26 🎉

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #28    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          60     68     +8     
  Lines        1898   2102   +204     
  Branches      121    133    +12     
======================================
+ Hits         1898   2102   +204

Impacted Files	Coverage Δ
docs/source/rules/examples/REQ-E004/tester.py	100% <100%> (ø)
docs/source/rules/examples/RES-E004/tester.py	100% <100%> (ø)
...er_spec_compatibility/walkers/changed_xnullable.py	100% <100%> (ø)
...les/removed_nullable_property_from_request_test.py	100% <100%> (ø)
...ility/rules/added_nullable_property_in_response.py	100% <100%> (ø)
.../rules/added_nullable_property_in_response_test.py	100% <100%> (ø)
tests/walkers/changed_xnullable_test.py	100% <100%> (ø)
...ty/rules/removed_nullable_property_from_request.py	100% <100%> (ø)
... and 6 more

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