CI for contract call

[tupui]

Two options:

1. Spin up the quick-start image, deploy a contract and test against it.
2. Check request to be sent to RPC and mock response from RPC.

Option 2 might be better at first. Try to use respx for that.

[tupui]

Use this to fund source accounts (will fund with 10K XLM on testnet):

import httpx
from stellar_sdk import Keypair

keypair = Keypair.random()
response = httpx.get("https://friendbot.stellar.org", params={"addr": keypair.public_key})