Implement Client entity

[ivlevdenis]

Docs - https://github.com/toggl/toggl_api_docs/blob/master/chapters/clients.md

[nifadyev]

According to migration guide, Clients related endpoints are moved into /me and /workspace/<id>/ paths. So related logic and entity structure should be refactored.

[nifadyev]

Rough plan:

• Necessary endpoints (available on free plan):
  • GET /me/clients
  • GET/POST /me/preferences/{client}
  • GET/POST {workspace_id}/clients
  • GET/PUT/DELETE {workspace_id}/clients/{client_id}
• Cover new logic with tests and type annotations