Can't close AsyncApiClient

[pranjalv123]

Describe the bug

Closing AsyncApiClient.close() calls ApiClient.close(), which calls self.rest_client.pool_manager.clear(), but the async REST client doesn't have a pool_manager.

To Reproduce

import datadog_api_client

client = datadog_api_client.AsyncApiClient(datadog_api_client.Configuration())

client.close()

Expected behavior The client should close open connections, but we get an AttributeError instead.