Support binary format strings (multipart/form-data)

[dbanty]

This PR adds support for the multipart/form-data body type and schemas with the string "binary" format.

Generally these are used together when uploading files. Generated clients will now have a types module in their models which will contain a File class. That class is used to contain all the data required to upload a file via multipart/form-data. The payload is the only required piece of file and can be any IO (e.g. a file pointer). Optionally, a file_name and mime_type can be provided as metadata to upload.

As part of this PR, to make testing easier and bring more clarity to coverage, the normal unit tests and end to end tests were separated. task check will run unit tests only, task e2e will regenerate the "golden_master" and run the end to end tests.

[codecov]

Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #45   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          573       597   +24     
=========================================
+ Hits           573       597   +24     

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

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