refactor: generalize aexn create contract mutation

[jyeshe]

What

• Generalize creation of AEX-N contract (aex9, aex141, ...)
• Detect NFT contracts

Why

Refs #629

This PR indexes data that allows listing NFT contracts along with metainfo and with the known pubkeys interact with them to provide endpoints such as NFT owner, owner balance, etc.

Validation steps

Unit tests

elixir --sname aeternity@localhost -S mix test test/ae_mdw/db/sync/contract_test.exs
elixir --sname aeternity@localhost -S mix test test/ae_mdw/db/sync/transaction_test.exs
elixir --sname aeternity@localhost -S mix test test/ae_mdw/db/aexn_create_contract_mutation_test.exs

Sync tests

1. Delete a contract by running on iex -S mix:
   iex(aeternity@localhost)1> pk = AeMdw.Db.Origin.pubkey({:contract, 4072415}); Database.dirty_delete(Model.AexnContract, {:aex9, pk})
2. Rename the migration mv priv/migrations/20220506104325_reprocess_contract_txs.ex priv/migrations/20220601104325_reprocess_contract_txs.ex
3. Change line 31 to: do_migrate(first_kbi..(first_kbi + 10))
4. Put an underscore on unused variable at line 27: _last_kbi
5. Run the iex -S mix to run the migration
6. Check contract creation with:

iex(aeternity@localhost)1> pk = AeMdw.Db.Origin.pubkey({:contract, 4072415}); Database.read(Model.AexnContract, {:aex9, pk})        
[
  {:aexn_contract,
   {:aex9,
    <<40, 189, 109, 123, 71, 195, 48, 205, 12, 53, 140, 168, 137, 35, 40, 4,
      104, 189, 96, 192, 213, 164, 48, 203, 240, 42, 163, 44, 94, 202, 204,
      232>>}, 4072415, {"Test", "TT", 0}}
]