fix: handle watcher .update() in case of async definition

[devamin]

Why

I was trying to use casbin-async-postgres-watcher with an AsyncEnforcer, and it appears that the enforcer watcher update call was sync, but the async-postgres-watcher is async, warned by the following

/Users/amin/Library/Caches/pypoetry/virtualenvs/uchat-oa-backend-dVRSYrvN-py3.12/lib/python3.12/site-packages/casbin/async_internal_enforcer.py:164: RuntimeWarning: coroutine 'AsyncPostgresWatcher.update' was never awaited
  self.watcher.update()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

What

I decided to refactor the AsyncInternalEnforcer to handle update in case of an async definition, while doing this I found out that I can reduce some code duplications.

To Reproduce

pseudo-snippet

from casbin.async_enforcer import AsyncEnforcer
from async_postgres_watcher import AsyncPostgresWatcher
from casbin.persist.adapters import FileAdapter

casbin_enforcer = AsyncEnforcer(adapter)

watcher = AsyncPostgresWatcher(host=HOST, port=PORT, user=USER, password=PASSWORD, dbname=DBNAME)
watcher.set_update_callback(casbin_enforcer.e.load_policy)

casbin_enforcer.set_watcher(watcher)

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