release-20.2: changefeedccl: allow configuring changefeeds to wait for all replicas

[HonoreDB]

Backport 1/1 commits from #67504.

/cc @cockroachdb/release

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… to ack

Changefeeds use Sarama's default config of RequiredAcks: WaitForLocal,
meaning it considers a write to Kafka successful once the leader node has
acked it. This carries a risk of dropped messages if the leader node acks
before replicating to a quorum of other Kafka nodes, then fails.
This PR allows overriding the config like so:

CREATE CHANGEFEED ... WITH kafka_sink_config='{"RequiredAcks": "ALL"}';

It's also now possible to increase throughput by decreasing consistency:

CREATE CHANGEFEED ... WITH kafka_sink_config='{"RequiredAcks": "NONE"}';

Release note (enterprise change): kafka_sink_config can include RequiredAcks

Closes #67368

Release justification: Urgently-requested feature, purely additive

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is