Allow bootstrap.servers to be provided via a PasswordProvider for Kafka Ingestion

[JulianJaffePinterest]

Description

Currently, the consumerProperties map in a KafkaSupervisorIOConfig can use PasswordProviders for the keystore, truststore and key properties, but bootstrap.servers must be a hard-coded String. Although bootstrap.servers is not a password, it would be helpful if the value for this property could be provided via a similar mechanism to a PasswordProvider. This would allow SupervisorSpecs to be robust to changes in the Kafka bootstrap servers in the event of a cluster migration or the like.

Motivation

Currently, if the bootstrap servers for a Kafka cluster change, running supervisor specs that refer to those bootstrap servers must be manually killed and relaunched with references to the new bootstrap servers. If a cluster operator has access to a separate system that can provide updated bootstrap server information, these manual operations can be avoided by borrowing the concept of a "Provider" from the PasswordProvider.

[gianm]

Maybe it'd make sense to allow this for any property?

[JulianJaffePinterest]

I agree that allowing most properties to be specified via a Provider makes sense. I think we'd have to figure out a good solution to #6666 or else users could encounter some very hard to debug Heisenbug-style errors though.

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