Add helper functions to sign authorization entries.

[overcat]

Closes #489

The following is an example.

  public void sign()
      throws SorobanRpcErrorResponse,
          IOException,
          AccountNotFoundException,
          PrepareTransactionException {
    SorobanServer sorobanServer = new SorobanServer("http://127.0.0.1:8000/soroban/rpc");
    Network network = Network.STANDALONE;
    String contractId = "CDCYWK73YTYFJZZSJ5V7EDFNHYBG4QN3VUNG2IGD27KJDDPNCZKBCBXK";
    KeyPair sourceKp =
        KeyPair.fromSecretSeed("SAAPYAPTTRZMCUZFPG3G66V4ZMHTK4TWA6NS7U4F7Z3IMUD52EK4DDEV");
    KeyPair opInvokeKp =
        KeyPair.fromSecretSeed("SAEZSI6DY7AXJFIYA4PM6SIBNEYYXIEM2MSOTHFGKHDW32MBQ7KVO6EN");

    InvokeHostFunctionOperation op =
        InvokeHostFunctionOperation.invokeContractFunctionOperationBuilder(
                contractId,
                "increment",
                Arrays.asList(Scv.toAddress(opInvokeKp.getAccountId()), Scv.toUint32(10)))
            .build();

    TransactionBuilderAccount source = sorobanServer.getAccount(sourceKp.getAccountId());

    Transaction transaction =
        new TransactionBuilder(source, network)
            .addOperation(op)
            .addPreconditions(
                TransactionPreconditions.builder()
                    .timeBounds(
                        new TimeBounds(BigInteger.ZERO, TransactionPreconditions.TIMEOUT_INFINITE))
                    .build())
            .setBaseFee(50000)
            .build();

    System.out.println(transaction.toEnvelopeXdrBase64());

    SimulateTransactionResponse simulateTransactionResponse =
        sorobanServer.simulateTransaction(transaction);
    // you need to check simulateTransactionResponse
    InvokeHostFunctionOperation invokeOp =
        (InvokeHostFunctionOperation) transaction.getOperations()[0];
    // add all auth to invoke host func op
    simulateTransactionResponse
        .getResults()
        .get(0)
        .getAuth()
        .forEach(
            auth -> {
              SorobanAuthorizationEntry e =
                  Auth.authorizeEntry(
                      auth, opInvokeKp, simulateTransactionResponse.getLatestLedger() + 3, network);
              invokeOp.getAuth().add(e);
            });

    Transaction preparedTx =
        sorobanServer.prepareTransaction(transaction, simulateTransactionResponse);
    preparedTx.sign(sourceKp);
    System.out.println(preparedTx.toEnvelopeXdrBase64());

    SendTransactionResponse sendTransaction = sorobanServer.sendTransaction(preparedTx);
    System.out.println(sendTransaction);
  }

[overcat]

Hi @sreuland, let's include this PR in the new release.

[sreuland]

for consideration in a later pr, could remove this duplicated docs by using @see instead:

/**
 * @see Auth#authorizeEntry(String, KeyPair, Long, Network) authorizeEntry
 */

and maybe re-evaluate the overloaded signatures of authorizeEntry and see if that can be narrowed?

[sreuland]

nice usage of functional interface!

[sreuland]

for consideration later, the duplicate test case variants for base64 and xdr object graph parameters could probably be reduced to just the base64 parameters, as that will by default exercise the xdr object path internally as well correct? would just. be less test code to maintain over time.

[overcat]

Yes, I will remove it later.

[sreuland]

great work, lot of complexity to auth, looks clean.