Declare an identify trait key

Registers the key ahead of any identify that sends it. Idempotent; engine keys are already declared. Keys are trimmed, at most 64 characters, and may not contain "/" or control characters. With allow_special_category, true lets identify store this special-category key (health_condition, medical_, religion, sexual_orientation, ethnicity, political_, trade_union, biometric_data, genetic_* and similar) and records who allowed it; false stops it. Other keys answer 400: secret and contact keys can never be allowed. Owner or admin only.