SQL tenant servers will receive access tokens from a dedicated auth broker service. A token encodes the tenant ID and an expiration timestamp. The pattern is roughly that a tenant is initiated with a valid token and can periodically contact the auth broker (using a valid token) to issue a new valid token with a larger expiration timestamp. For simplicity, the auth broker will live in the main repo and comes bundled with ./cockroach, though we will also allow building it as a small stand-alone binary (from ./pkg/cmd/authbroker).

See #47898.