Use assignment label to select which BrokerCell should service a Broker

[grantr]

Problem
In #866 we'll define a label or annotation that assigns a Broker to a BrokerCell. The BrokerCell controller should use this value to decide which BrokerCell should service a broker, and in which targets configmap the broker's targets should be contained.

Exit Criteria
Brokers are serviced by the BrokerCell identified in their metadata

Additional context (optional)
Not strictly blocked by #866, but blocked by the decision about which key will be used to denote this assignment.

[grantr]

Also part of this: adding assignment awareness to the Broker controller so it can decide which BrokerCell determines readiness and Broker address.

[github-actions]

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