chore: upgrade gapic-generator-python to 0.46.3

[parthea]

This PR is the result of manually running the following commands in order to pull in the latest changes from googleapis-gen which includes gapic generator version 0.46.3:

1. Download the latest docker image for owlbot-cli

docker pull gcr.io/repo-automation-bots/owlbot-cli:latest 

2. Run the owlbot-cli to copy the generated client from googleapis/googleapis-gen

docker run --rm --user $(id -u):$(id -g) -v $(pwd):/repo -w /repo gcr.io/repo-automation-bots/owlbot-cli:latest copy-code 

3. Download the latest docker image for owlbot-python

docker pull gcr.io/repo-automation-bots/owlbot-python:latest 

4. Run the post processor image

docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo gcr.io/repo-automation-bots/owlbot-python:latest 

Other features/fixes include:

fix: add async client to %name_%version/init.py
chore: add autogenerated snippets
chore: remove auth, policy, and options from the reserved names list
feat: support self-signed JWT flow for service accounts
chore: enable GAPIC metadata generation
chore: sort subpackages in %namespace/%name/init.py

[busunkim96]

Please add packaging>=14.3 to setup.py and packaging==14.3 and google-auth=1.24.0 to testing/constraints-3.6.txt.

[parthea]

Done.