[BUG] Plugin feature flags failing after refactor in core

[harshavamsi]

Describe the bug

With #17611, we refactored the way feature flags work. I'm trying to onboard the neural search plugin to use the new feature flag constructs but it does not work today because we check against all registered feature flags in core before we set the value. Since plugins do not register their feature flags to core this breaks while checking

        void set(String featureFlagName, Boolean value) {
            for(Setting<Boolean> ff : this.featureFlags.keySet()) {
                if (ff.getKey().equals(featureFlagName)) {
                    this.featureFlags.put(ff, value);
                }
            }

        }

Related component

Plugins

To Reproduce

Trying to run tests in neural search

Expected behavior

Tests should pass

Additional Details

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
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[andrross]

@finnegancarroll FYI

[vibrantvarun]

@finnegancarroll Can we have a functionality where we can set a feature flags from the plugin?

[finnegancarroll]

Hi @vibrantvarun , pr here, validating against neural search test failure.

[finnegancarroll]

Closed with: opensearch-project/neural-search#1271 (comment)
No remaining plugins register new feature flags.