Improve supervisor repr for debugging

[zanieb]

import concurrent.futures
import threading

import pytest

from prefect._internal.concurrency.supervisors import AsyncSupervisor, SyncSupervisor


def fake_submit_fn(__fn, *args, **kwargs):
    future = concurrent.futures.Future()
    future.set_result(__fn(*args, **kwargs))
    return future


def fake_fn(*args, **kwargs):
    pass


supervisor = SyncSupervisor(submit_fn=fake_submit_fn)
print(repr(supervisor))

supervisor.submit(fake_fn, 1, 2)
print(repr(supervisor))

<SyncSupervisor submit_fn='fake_submit_fn', owner='MainThread'>
<SyncSupervisor submit_fn='fake_submit_fn', submitted='fake_fn', owner='MainThread'>

vs previously

<SyncSupervisor(id=140242493845264, owner=8633447104)>
<SyncSupervisor(id=140242493845264, owner=8633447104)>

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