use `nostr-zapper` crate to implement `sell::verify_zap`

[plebhash]

the mine JSON-RPC method allows a buyer to submit:

• an event to be mined
• the desired PoW difficulty
• the zap paying for the PoW

$ curl -s -X POST \
     -H "Content-Type: application/json" \
     -d '{
           "jsonrpc": "2.0",
           "method": "mine",
           "params": {
             "event": {
               "pubkey": "98590c0f4959a49f3524b7c009c190798935eeaa50b1232ba74195b419eaa2f2",
               "created_at": 1668680774,
               "kind": 1,
               "tags": [],
               "content": "hello world"
             },
             "difficulty": 15,
             "zap": "f481897ee877321783bb76133622b3cc344d691bb79cd6be88f44e819c3b2306"
           },
           "id": 1
         }' \
     http://localhost:1337

it is assumed that the buyer has previously called the quote JSON-RPC method, so they know how many sats they should send in the zap.

but maybe they send an invalid zap, or the zap does not have enough sats.

so the JSON-RPC handler needs to validate the zap before it starts mining: https://github.com/plebemineira/notemine_hw/blob/ade25cbb4b75fb8d05e56a10c7a211dae454191c/src/lib/service.rs#L101-L111

currently, verify_zap has a dummy placeholder implementation: https://github.com/plebemineira/notemine_hw/blob/759177453006f8b9a0ae6c18c51c342ebc491dfa/src/lib/sell.rs#L7-L25

this implementation should leverage nostr-zapper crate to actually verify if:

• zap is valid
• zap contains more sats than pow_price

only if those two verifications are succesful, should we start mining

[plebhash]

https://nostr.how/en/zaps