[Ballista] Create "self-contained" mode in client library #588

From https://github.com/ballista-compute/ballista/issues/637

It would be nice to have a truly self-contained mode for Ballista, where you could run some queries without needing any additional processes. This would be an opt-in feature of the client crate.

Right now, you need to launch an executor in "standalone mode" (i.e. with an in-proc scheduler), which is not ideal. Once we add this, we could remove the "standalone mode" in the executor.

See this for context