[Refactoring] Move MLFeatureEnabledSetting to Common

[austintlee]

For other subprojects in ml-commons to have feature flags, we need common classes in org.opensearch.ml.settings to move out of the 'plugin' subproject and into 'common'. Since the plugin project takes dependency on all other subprojects, leaving MLFeatureEnabledSetting in 'plugin' leads to a circular dependency when other subprojects need to use this class.

[mingshl]

when the next feature flag gets in, need to pick this ticket up.