Announce the payer's handoff to the provider (kessai.initiated)

Frank Mablyrequested to merge
issue/kessai-3611852:3611852-announce-the-payers into 1.x

Merge request for #3611852.

Adds a kessai.pay handoff route + PaymentHandoffController that fires the new PaymentEvents::INITIATED when the payer deliberately hands off to the provider, so a consumer can bracket the round-trip. initiate() stays event-free.

Merge request reports