txnbuild: add helpers for constructing transactions to send payments to contracts

[tamirms]

Currently it's very easy to use txnbuild to send a payment to another account. However, it's not as straightforward to send payments to recipients which are smart contracts.

We should update txnbuild to support constructing SAC payments. The auth and resource / fee values should be hardcoded so that we can construct the the invoke host operation without requiring RPC's simulateTransaction endpoint.

See lightsail-network/java-stellar-sdk#677 and StellarCN/py-stellar-base#1023 for examples of how this functionality was implemented for the java and python SDKs

[tamirms]

cc @marcelosalloum

[Shaptic]

Opened stellar/js-stellar-base#792 to track JS work for this.