Returns the encryption key that would be sent in webhooks for the given entity.
This allows counterparties to proactively fetch encryption keys without waiting
for a RequirePresentation webhook.

V1 VASP Handling:
For entities using the TRNow protocol (V1 VASPs with did:ethr DIDs), this endpoint
automatically returns the Gateway VASP's encryption keys instead of the entity's own keys,
since V1 VASPs don't have their own encryption keys. The response includes a gatewayDID
field to indicate this.

Performance Optimization:
For did:web entities in the local database, encryption keys are resolved directly
without Network API calls, enabling fast response times for local V2 entities.

Key Selection Priority:
Uses the same priority algorithm as webhook sending:

1. Keys ending with "#pii" (highest priority)
2. Keys ending with "#notabene-pii"
3. Keys with type "X25519KeyAgreementKey2019"
4. First available key (fallback)

Response is cached via Veramo's DID resolution cache (5 minute TTL).