build-clients: check if accounts are already created/funded

[chadoh]

Follow-up to #42

In the "creating keys for _____" step, we want to speed things up and keep them more predictable by skipping key-generation and account-funding if the account already exists.

We should maybe ship this as an upstream update in soroban keys generate, which is what is used by build_clients.rs. I've created an issue there, to see how everyone feels about this logic being included in the main CLI:

• keys generate: don't overwrite existing account stellar/stellar-cli#1380

[BlaineHeffron]

Currently a workaround is in place but once stellar/stellar-cli#1380 and stellar/stellar-cli#1389 are merged into an updated release, we can remove it.