invalidate reputation to prevent sybil attack

[brenzi]

as specified in implementation Option 1 here: https://github.com/encointer/encointer-node/issues/300

pallet side:

• needs: https://github.com/encointer/pallets/issues/303

oracle side:

• query reputation commitments for the purpose of nostr badges
• when verifying proff of attendance, also check that it hasn't been used for the same purpose already

[brenzi]

see more detailed requirement - solution matching here: https://github.com/encointer/pallets/issues/303#issuecomment-1531684746