#6 Add OAuth authorization_url

[nstillman-te]

Part of #6.

Adds client.oauth.authorization_url() to build the OAuth2 authorization URL (GET /oauth) for the Authorization Code flow with PKCE. Callers redirect the user to the returned URL; no HTTP request is made by the client. Optional params: state, scope, username. Unit tests assert the built URL contains the correct query params (no requests-mock: method does not perform a request).

Checklist when adding a new endpoint

• Added new endpoint to the README.rst
• 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/ — N/A (endpoint returns a URL string, no JSON)
• Written tests for the endpoint (unit tests for URL construction; no VCR — method does not perform a request)
• Added the endpoint to CHANGELOG.rst in the To be released section