Read the account tier, quota period, and purchasable plans

Read billing state and return {tier, period, payment_method_attached, plans, checkout}. This call costs nothing quota-wise. Call it after authentication, after quota_exceeded, or after a human confirms checkout. Poll no more than once per five minutes, with at most six checks. Recover from invalid_key at the web key page; for billing_unavailable or rate_limited, wait retry_after_sec and retry once.