Updates for Google Photorealistic 3D Tiles

[ggetz]

This PR

• adds two basic Sandcastles demonstrating Google's Photorealistic 3D Tiles.
• addscreateGooglePhotorealistic3DTileset to create a 3D tileset streaming Google Photorealistic 3D Tiles.
• adds GoogleMaps for managing credentials when loading data from the Google Map Tiles API.
• updates RequestScheduler.requestsByServer, to allow for more simultaneous requests to this API.
• updates the release guide with instructions for updating the api key each release

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[jjhembd]

Looks good, thanks @ggetz!