Allow --source-account to be a public key

[Ifropc]

What problem does your feature solve?

With added --sign-with and tx sign functionality (#1406) there should be a way to originate transaction from account we don't know a secret key for. Currently, we must provide a keypair for --source-account in order to form transaction.
Previously, it was always signed by the provided keypair (unless --build-only or --simulate-only were passed. However, with this flags the transaction could never be submitted, as there was no standalone sign functionality).
In turn, now we can write code stellar contract <command> --source-account A --simulate-only | tx sign --sign-with-key B. This will set transaction source account to A, while signing it with B. We no longer need to know private key of A, because it never signs transaction, while formed transaction is still valid (and can be submitted to the network successful under correct signer weights).

What would you like to see?

--source-account <public_key>

What alternatives are there?

Always importing private key for --source-account