feat: Support for reusing existing MongoDB client

[omseervi098]

Currently, casbin_pymongo_adapter always creates a new MongoDB client by parsing the connection URL internally. This makes it difficult to reuse an already-initialized MongoClient that may be shared across the application.

Allow passing an existing MongoClient as an optional argument to the adapter, for example:
adapter= Adapter(client=mongo_client, db_name="casbin")

while keeping existing one