feat: Support UserIdentificationType

[BrandonStalnaker]

Summary

• Adds userIdentificationType so AppsFlyer’s customer user ID can follow MPID, CustomerId, or legacy behavior, including launch/identity-callback sync and commerce af_customer_user_id via customerIDForAppsFlyer:.
• setUserIdentity is updated so MPID mode ignores customer-ID identity for AppsFlyer’s customer user ID but still returns success, while legacy and CustomerId mode keep forwarding that identity to AppsFlyer.

Testing Plan

• Was this tested locally? If not, explain why.
• The mock now records writes to AppsFlyer’s customerUserID property (the real SDK path) so tests can assert when the kit sets it.
• New tests cover setUserIdentity for legacy, CustomerId, and MPID modes plus email and unsupported identity types, alongside the existing manualStart / didBecomeActive checks.

Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/REPLACEME