Adding import_pgn endpoints

[dat-boris]

Hi there,

Thanks for the project! This small PR add the study/ImportPGN endpoint.

https://lichess.org/api#tag/Studies/operation/apiStudyImportPGN

Testing with:

  session = berserk.TokenSession(API_TOKEN)
  client = berserk.Client(session=session)
  resp = client.studies.import_pgn(
      study_id=STUDY_ID,
      chapter_name="test",
      pgn=PGN,
      orientation="white",
  )
  for chapter in resp:
      print(chapter)

Returning the imported chapters.

{'id': 'oavP10Ig', 'name': 'test'}

Checklist when adding a new endpoint

• Added new endpoint to the README.md
• Ensure that my endpoint name does not repeat the name of the client. Wrong: client.users.get_user(), Correct: client.users.get()
• Typed the returned JSON using TypedDicts in berserk/types/, example
• Tested GET endpoints not requiring authentification. Documentation, example
• Added the endpoint and your name to CHANGELOG.md in the To be released section (to be created if necessary)

[kraktus]

Thanks for the PR! I've added/changed some more types, otherwise great work!