smplx_sdk: add wrapper over Transaction for fetch_transaction

[ikripaka]

• This PR suggests a bug fix and I've added the necessary tests.
• This PR introduces a new feature and I've discussed the update in an Issue or with the team.
• This PR is just a minor change like a typo fix.

---

• Add simplex::transaction::Transaction type to wrap behaviour over Transaction. In this way, we need to invoke get_unblinded_out or get_explicit_out to obtain a UTXO object from elements::Transaction
• Changes code in the following way.
  Before:

let lending_utxo = provider.fetch_scripthash_utxos(&lending.get_script_pubkey())?[0].clone();

    let lending_creation_tx = provider.fetch_transaction(&lending_creation_txid)?;

    let first_parameters_nft_utxo = UTXO {
        outpoint: OutPoint::new(lending_creation_txid, 1),
        txout: lending_creation_tx.output[1].clone(),
        secrets: None,
    };
    let second_parameters_nft_utxo = UTXO {
        outpoint: OutPoint::new(lending_creation_txid, 2),
        txout: lending_creation_tx.output[2].clone(),
        secrets: None,
    };

After:

let txid = finalize_and_broadcast(&context, &ft)?;
    provider.wait(&txid)?;

    let lending_utxo = provider.fetch_scripthash_utxos(&lending.get_script_pubkey())?[0].clone();

    let lending_creation_tx = provider.fetch_transaction(&lending_creation_txid)?;

    let first_parameters_nft_utxo = lending_creation_tx.get_explicit_out(1)?;
    let second_parameters_nft_utxo = lending_creation_tx.get_explicit_out(2)?;