fix: add google-auth as a direct dependency

[parthea]

google.auth is used in this repository and should be added as a direct dependency so that the minimum version can be stated and tested
https://github.com/search?q=repo%3Agoogleapis%2Fpython-firestore%20google.auth&type=code

The setup.py from the autogenerated code is not used in this repository hence this PR to apply the changes manually.

python-firestore/setup.py

Lines 33 to 39 in 3a572b5

	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.*",
	"google-cloud-core >= 1.4.1, <3.0.0dev",
	"proto-plus >= 1.22.0, <2.0.0dev",
	"proto-plus >= 1.22.2, <2.0.0dev; python_version>='3.11'",
	"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",
	]