fix: merge feature flags with returned values from API to enrich flag availability in config

[estephinson]

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Summary

• Ensures feature flags that have been returned by the API are available within the context of netlify-config

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[estephinson]

Can we add some tests?

I can't see an easy path to getting tests around this.

I've refactored to move the logic into getSiteInfo, but will be adding a single unit test to assert on that