Add Google OAuth2 example

[frankvp11]

Adds the Google OAuth2 example that I created here
Unfortunately, I'm not very skilled with readme's, so I used some images and text where I could. I ask that you guys fix that (if you feel it's necessary).
Let me know what you guys think.

[rodja]

@frankvp11 I finally found the time to spend some time on this PR. While your code worked I took the liberty to refactor and simplify it a bit. Please have a look and tell me what you think. Also it's interesting to compare it with the rework I've done to #3244, where I applied a lot of learnings from this PR.

[frankvp11]

Glad to hear you were able to understand my code, and make use of it in your example as well. It looks a good bit cleaner now.

[rodja]

It would be great if we can get rid of the guide_images and simplify the README.md. @frankvp11 is there not a document at Google which describes the steps? I think it's much better if we do not need to maintain our own documentation. For one-tap-auth I found such a description:

nicegui/examples/google_one_tap_auth/main.py

Line 11 in a366de0

# See https://developers.google.com/identity/gsi/web/guides/get-google-api-clientid#get_your_google_api_client_id.

Also the example should be added to https://github.com/zauberzeug/nicegui/blob/main/website/examples.py

[frankvp11]

By the looks of it, that covers the exact same steps (this link, that you provided). I think it wouldn't be a bad idea to just remove the README and images all together and add the comment to the main.py itself.
What do you think?

[frankvp11]

Were there any more updates required from my end?