Add public API for blocking on a run until it's complete

[augray]

People may want to launch a run and then wait to see if it succeeds (ex: if using a Sematic pipeline in CI). This can be done as follows:

import time
from sematic import api_client
from sematic.abstract_future import FutureState

def block_until_done(run_id: str, poll_interval_seconds: int):
    keep_going = True
    while keep_going:
        run = api_client.get_run(run_id)
        state = FutureState[run.future_state]
        keep_going = not state.is_terminal()
        time.sleep(poll_interval_seconds)
    if state != FutureState.RESOLVED:
        raise RuntimeError(f"Run {run_id} finished in state {state}")

However, this uses non-public APIs. We should expose something like block_until_done as a public API.