Fixes google-cloud-sdk-datastore-emulator install issue in e2e test

[dwsmart]

Fixes the failing e2e test in the workflow due to google-cloud-sdk-datastore-emulator not being installed. Addresses #682

Installs google-cloud-sdk & google-cloud-sdk-datastore-emulator from the Google Cloud repository instead, as google-cloud-sdk-datastore-emulator package not available in the ubuntu default.

It does make the test slow, but it works.

Also adds documentation updates for the GET /invalidate API for clearing the entire cache. addressing #691

[dwsmart]

Note, the first test run seemed to fail on one integration test (which remains unchanged), but I think that was general actions flakiness, it just timed out.