fix: parser / add summary attribute to endpoint model

[p1-ra]

This PR add an optional summary attribute to the Endpoint model.

By reading the specs the field summary and description seem to be ~six of one, half-a-dozen of another.
Thus, merging the two fields in the already existing description attr of the Endpoint is probably a bad idea.

In my case I use it with a custom template to generate docstring. An other solution will be to use the summary value of the scheme as a fall-back value for the Endpoint description; but their is maybe someone out there that need these two fields.

[codecov]

Codecov Report

Merging #404 (503a0ed) into main (518b501) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #404   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           46        46           
  Lines         1530      1531    +1     
=========================================
+ Hits          1530      1531    +1     

Impacted Files	Coverage Δ
openapi_python_client/parser/openapi.py	100.00% <100.00%> (ø)

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[dbanty]

There's potential for the same issue here where someone realizes that summary isn't being used by any internal code and removes it... but I think the answer to that is to document the entirety of this class which is out of scope for this PR.