Place anchor based on XY screen coordinates and update ARCore SDK to 1.35.0

[dnkoulouris]

A pull request that adds functionality to place an anchor at the X,Y coordinates of the screen.

I also did some updates to ARCore libraries, in order to have the most recent available versions.

Just call:

// Places anchor in given 2D coordinates (starting from the top-left side of the screen)
placeBasedOnScreenCoordinates(double x, double y)

and an Anchor will be placed at the exact spot

[CariusLars]

Hey @dnkoulouris, thanks for this PR! Have you tested to make sure all functionalities of the example app work on both Android and iOS?

If this is the case, could you please:

1. Merge in main of this repo to resolve the merge conflict
2. Write an entry for your feature and fixes into changelog.md
3. Set a new minor version number in the readme and the pubspec.yaml

Happy to merge after that :)

[dnkoulouris]

Just performed the aforementioned changes. I also changed the cloud anchor setup file, indicating the requirement of enabling the new ARCore API (The old one, Cloud Anchors API, is deprecated) from the Google Cloud Console.

[JeroenMoonen]

Nice contrib @dnkoulouris, waiting for this release!

[tonyhart7]

lgtm