feat: Introduce compatibility with native namespace packages

[parthea]

This PR:

• Migrates code to use native namespace packages similar to feat: Introduce compatibility with native namespace packages google-auth-library-python#1205. Also see https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages
• Removes usage of pkg_resources to prepare for adding python 3.12 support
• Adds a version module to google/cloud/dns

To obtain the version at runtime, use google.cloud.dns.__version__

(py39) partheniou@partheniou-vm-3:~/git/python-dns$ python3
Python 3.9.16 (main, Mar  2 2023, 17:52:22) 
[GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import google.cloud.dns
>>> google.cloud.dns.__version__
'0.34.2'

[parthea]

Adding do not merge as there is an upcoming release freeze. This should be merged after the release freeze ends on November 29th.