[v24.3.x] datalake/configs: rework iceberg enablement configs (Issue #24276)

[bharathv]

Backport #24222 to branch v24.3.x

Backports Required

• [ ] none - not a bug fix
• [x] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.3.x
• [ ] v24.2.x
• [ ] v24.1.x

Release Notes

• none

[vbotbuildovich]

non flaky failures in https://buildkite.com/redpanda/redpanda/builds/58649#01935a28-06f2-406b-908e-5ae1024fcfba:

"rptest.tests.data_transforms_test.DataTransformsLoggingTest.test_tunable_configs"

[vbotbuildovich]

Retry command for Build#58649

please wait until all jobs are finished before running the slash command

/ci-repeat 1
tests/rptest/tests/data_transforms_test.py::DataTransformsLoggingTest.test_tunable_configs