Add broadcasts.get_by_user endpoint to get broadcasts created by a user

[DoraFgr]

This PR is a small part of #6 and implements broadcasts.get_by_user() endpoint to retrieve broadcasts created by a specific user

Summary:

• Implement client.broadcasts.get_by_user() to fetch paginated broadcasts by username
• Add BroadcastsByUser TypedDict for response typing
• Add test with VCR cassette validation

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)

[DoraFgr]

There seem to be an issue with the CI test. I opened #150 to futher discuss the issue.

[DoraFgr]

I don’t seem to have the required permission to rerun the CI. If someone can rerun the jobs when reviewing, that would be great.

[kraktus]

thanks! clean PR 👍