Do not require source account when reading contract.

[fnando]

What

Before:

$ stellar contract read --id hello --network testnet
error: the following required arguments were not provided:
  --source-account <SOURCE_ACCOUNT>

Usage: stellar contract read --source-account <SOURCE_ACCOUNT> --id <CONTRACT_ID> --network <NETWORK>

For more information, try '--help'.

After:

$ stellar contract read --id hello --network testnet
LedgerKeyContractInstance,"{""hash"":""4e1ff745c9f12141dca37e5116e994594812c23c091650c5756a2de7857423d3""}",329089,2402688

Why

The source account was being required just because of how clap's flattening work.

Known limitations

N/A

[willemneal]

Just one suggestion