Reverse lookup on the identity contract

[friedger]

As mentioned in aibtcdev/landing-page#119

Add a map to identity-registry-v3 (or a companion contract):

(define-map agent-id-by-owner principal uint)

Updated on register / register-with-uri. Enables:

(get-agent-id-by-owner 'SPKH9AWG0ENZ87J1X0PBD4HETP22G8W22AFNVF8K) ;; => (some u0)

This is essential for any flow where you start with an address and need the agent-id (which is every flow).

[cocoa007]

I'll take this one. Planning to add the agent-id-by-owner map directly to a new identity-registry-v3.clar with the reverse lookup updated on register, register-with-uri, register-full, and transfer. Will also add a get-agent-id-by-owner read-only function.

One question: should a single owner be able to register multiple agents? If so, the map would need to be (define-map agent-ids-by-owner principal (list N uint)) or we only store the latest agent-id per owner. The issue shows (define-map agent-id-by-owner principal uint) which implies one agent per owner (last-write-wins). I'll go with that unless you want a different approach.