[task scheduling] add `TaskServer.execute_task_run`

[zzstoatzz]

this PR adds an execute_task_run method that runs a TaskServer for the duration of a single task run, which is very convenient for testing and other one-off use of the TaskServer

after some engine / task_runner exploration / learning with @abrookins, in the short term we were able to make a couple small resource management improvements

Notes (afaict)

• without calling .result() on the PrefectFuture we get back from submit_autonomous_task_run_to_engine, wait is never called on the ConcurrentTaskRunner, so we never exit the EngineContext (whereas with SequentialTaskRunner, we needn't call .result() because we're not waiting for an Event to be set)
• it may be prudent to use wait_for_task_runs_and_report_crashes in some way with the PrefectFuture instances we get back in TaskServer instead of the current approach of start_soon for each PrefectFuture._result

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