List TikTok ad identities

The identities an ad on this TikTok advertiser may run as (the profile shown on the ad): the advertiser's own TikTok accounts (TT_USER), Business Center authorized accounts (BC_AUTH_TT) and custom brand identities (CUSTOMIZED_USER). Pass the chosen identityId on POST /v1/ads/create or POST /v1/ads/boost. Spark-post identities (AUTH_CODE) are not listed; a Spark code creates its own.