Implement CurrentUser entity

[ivlevdenis]

https://github.com/toggl/toggl_api_docs/blob/master/chapters/users.md

[nifadyev]

According to migration guide, most endpoints for current users are available at /me path. So Users related repository should be refactored

[nifadyev]

Rough plan:

• Add check for billing plan. Request and response fields set depend on it, as well as endpoints availability
• Write explicit note about using UTC timezone for datetime (until some type of package config with user timezone is implemented)
• Necessary endpoints (available on free plan):
  • GET/PUT /me - Returns details for the current user. Returns huge response, which requires a lot of pydantic schemas for mostly all toggl entities
  • GET /me/features
  • GET/POST /me/preferences
• Cover new logic with tests and type annotations