feat: Add the protected apiVersion property

[Hectorhammett]

Add the protected apiVersion property to the Resource class to support the new api version header.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[bshaffer]

It's a small feature, but I'd still like to see a test for it!

[Hectorhammett]

@bshaffer Added!