feat: expose library version at googleapiclient.__version__

[joinsion]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [V] Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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• [V] Appropriate docs were updated (if necessary)

Fixes #1622 🦕

[parthea]

Thanks @joinsion!