Add google-auth as a direct dependency of generated clients

[parthea]

google.auth is imported in the generated code but is not declared as a direct dependency in setup.py. google-auth is pulled in transitively via google-api-core.

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

Line 25 in cd0325b

from google.auth import credentials as ga_credentials # type: ignore

gapic-generator-python/gapic/templates/setup.py.j2

Lines 32 to 36 in cd0325b

	dependencies = [
	"google-api-core[grpc] >= 1.34.0, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*,!=2.8.*,!=2.9.*,!=2.10.*",
	"proto-plus >= 1.22.3, <2.0.0dev",
	{# Explicitly exclude protobuf versions mentioned in https://cloud.google.com/support/bulletins#GCP-2022-019 #}
	"protobuf>=3.19.5,<5.0.0dev,!=3.20.0,!=3.20.1,!=4.21.0,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",

https://github.com/googleapis/python-api-core/blob/5922f2b8f382ee1fd9b69fafb2eed39db93b60fb/setup.py#L34

This led to issue googleapis/google-cloud-python#12254 because the minimum version of google-auth is not explicitly stated or tested.