Implemented client.studies.get_by_user

[JAMoreno-Larios]

Implemented /api/study/by{username} under clients.studies.list_by_username to get an iterator that yields the metadata of all studies of a given user.

Now, the interesting question: how can we write a test for this? There is no way to programmatically add new studies to a given user, and the studies that can be seeded with lila-db-seed are from users that do not have an account in that database.

Should I manually clone those studies and record the responses using pytest-recording? I'd appreciate your feedback

Checklist when adding a new endpoint

• Added new endpoint to the README.md
• Ensured 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
• Written tests for 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)