Add support for the following parameters in async REST

[ohmayr]

The following parameters need to be supported in asynchronous REST transport class for feature parity with synchronous REST:

"google.api_core.client_options.ClientOptions.credentials_file",
"google.api_core.client_options.ClientOptions.scopes",
"google.api_core.client_options.ClientOptions.quota_project_id",
"google.api_core.client_options.ClientOptions.client_cert_source",
"google.api_core.client_options.ClientOptions.api_audience",