Adds a cluster feature called TEST_ONLY_FEATURE. It can only be marked
as supported via error injection "features_enable_test_feature", which
can be enabled on node startup via CLI option or YAML configuration.
The purpose of this cluster feature is to simulate upgrading a node to a
version that supports a new feature. This allows us to write tests which
verify that the cluster feature mechanism works.
The fact that TEST_ONLY_FEATURE can only be enabled via error injection
should make it impossible to accidentally enable it in release mode and,
consequently, in production.
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