requestId is assigned even in non-payments activity with the nostr event id

[miche1e]

Activities have a field called requestId which is specific to the payments protocol and often fetched from request.content.requestId.
In any other case (such as auth requests) this field is valued from the nostr eventId fetched from event.eventId.
Since the nostr event id has no purpose (beside app-side events deduplication which is already fixed by the new tasks architecture) this field has no utility.
We can:

1. add a requestId for others requests ( -> time consuming, useless feature)
2. make the requestId optional and only be valued by payments like invoice field ( -> quick but without measurable improvements)
3. leave it as it is ( -> incoherent and confusing)