Implement Tag entity

[ivlevdenis]

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

[nifadyev]

Current state is:

• Invalid url path (should be either https://api.track.toggl.com/api/v9/workspaces/{workspace_id}/tags or https://api.track.toggl.com/api/v9/me/tags, now - https://api.track.toggl.com/api/v9/tags)
• No DELETE Method
• Actual requests to API are not tested yet, so its is impossible to say if pydantic model is valid

[nifadyev]

Rough plan

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