Set Boulder-Requester header for existing accounts (#5291)
Set Boulder-Requester header for existing accounts (#5291)

In wfe2, set the `Boulder-Requester:` header from the NewAccount API
endpoint when returning a pre-existing account.

This header is already set when this endpoint creates a new account,
but was omitted when returning an existing account.
letsencrypt/boulder,jsha/boulder,bmw/boulder,jcjones/boulder,letsencrypt/boulder,jcjones/boulder,bmw/boulder,jsha/boulder,jcjones/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,bmw/boulder