Create a user API key

Create a new bencher_user_* API key for a user. The plaintext key is only returned once in the response. Only the authenticated user themselves and server admins have access to this endpoint. This endpoint cannot be called with a user API key: a key cannot be used to create another key, so revoking a key also cuts off any credentials that could have been derived from it. Authenticate with a JWT (user session token) instead.