kafka: Disable FF if rack awareness is disabled

[michael-redpanda]

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x
• v23.1.x

Release Notes

Bug Fixes

• Have fetch handler ensure rack awareness is enabled before performing follower fetching

[emaxerrno]

Makes sense, because FF is enabled by default w/ client settings. Wonder if the degenerative case is every computer in a single rack has rack-labels as a way to have catch up workloads .... so instead of "rack-awareness:true" it is a function of rack-awareness-label.size() == 0 ... does that make sense? like should we drop the rack-awareness true and always make it true if rack-awareness-labels are existing.

in other words should the flag be only for disabling rather than for enabling

[vbotbuildovich]

new failures in https://buildkite.com/redpanda/redpanda/builds/43316#018c92a5-ef84-4540-bf53-8b883e38e6d0:

"rptest.tests.cloud_retention_test.CloudRetentionTest.test_cloud_retention.max_consume_rate_mb=20.cloud_storage_type=CloudStorageType.ABS"

[michael-redpanda]

#15913

[vbotbuildovich]

/backport v23.3.x

[vbotbuildovich]

/backport v23.2.x