Get rid of drf yasg. Implement OpenAPI 3

[mikhaillazko]

• Implement function for generating OpenAPI 3 spec
• Rewrite tests that improve stability
• Refactor structure of package winter_openapi
• Update poetry version

[mofr]

It closes #241

[mofr]

To do:

• [x] Try with code generators (at least python-nextgen and typescript-axios)
  • [x] Some endpoints are lost (order + groupby issue)
  • [x] requestBody parameter generated with another name (previously it was named "data")
  • [x] orderBy parameter has wrong type
  • [x] Any typing is not valid
  • [x] tags are missing (need to keep the same behavior as drf_yasg for now)
• [x] Add openapi validator to tests: https://github.com/p1c2u/openapi-spec-validator
• [x] Add openapi validator as an optional step to generate_openapi function
• [x] Check "Try it out" function works in Swagger UI
• [x] Decide tags and servers parameters purpose
• [x] supplyshift_identity_winter should be updated after merging because depends on winter_openapi
• [x] Update winter-getting-started