Implement User management

[nifadyev]

Necessary endpoints (available on free plan):

• PUT /organizations/{organization_id}/users/{user_id}

• PATCH /organizations/{organization_id}/users

• DELETE /organizations/{organization_id}/users/leave

• GET/POST/PATCH /workspaces/{workspace_id}/project_users

• PUT/DELETE /workspaces/{workspace_id}/project_users/{user_id}

• GET/POST/PATCH /workspaces/{workspace_id}/project_users

• GET/PUT /workspaces/{workspace_id}/users

• GET/PATCH/DELETE /workspaces/{workspace_id}/workspace_users

• Cover new logic with tests and type annotations

API documentation