feat: Handle analytics

[gewenyu99]

We have no good way of sending feature flag and custom properties to LLM Gateway, now we do. This should also unlock experiments for things that depend on LLMA.

Feature flags

The Wizard will send all feature flags with wizard prefix. Adds a getAllFlagsForWizard that fetches and caches all flags. In the future, we can reused this for everything to make sure we only evaluate flags once.

The LLM gateway will capture $feature/feature_flag_name: value in properties, this can be used for experiments.

Custom properties

We can only set custom properties once at the beginning of a run. What I propose is to have the concepts of variants controlled by a wizard-variant multivariate feature flag

Then, we can set what ever custom properties like this in constants.ts

/** Variant key -> metadata for wizard run (VARIANT flag selects which entry to use). */
export const WIZARD_VARIANTS: Record<string, Record<string, string>> = {
  base: { VARIANT: 'base' },
  subagents: { 
		VARIANT: 'subagents',
		FOO: 'BAR'
		POTATO: 'tomato'
 },
};

For example, if subagents is selected, all LLM gateway captures will include the custom properties variant: subagents, foo: bar, potato: tomato as custom properties.

See LLM gateway PR for more:
PostHog/posthog#49120

[github-actions]

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