Expose TikTok's native follow/community-interaction metric on AdMetrics

TikTok campaigns optimized for followers (optimizationGoal: "FOLLOWERS") come back with actions: {} and conversions: 0 across every ads endpoint (/v1/ads/tree, /v1/campaigns/{id}/analytics, /v1/ads/{id}/analytics) — there's no way to read how many follows a FOLLOWERS-goal campaign actually generated.

Confirmed with Zernio support (Ana) that this is a normalization gap on Zernio's side, not a TikTok API limitation: TikTok's own Ads reporting API supports a native follow metric for engagement-objective campaigns, it's just not mapped into AdMetrics yet.

Ask: normalize TikTok's native follow/community-interaction metric into AdMetrics, e.g. as an actions-style key (matching how Meta's action types are already exposed), so optimizationGoal: "FOLLOWERS" campaigns have a matching result field to read.